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* git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-2.6-fscache: (41 commits)
  NFS: Add mount options to enable local caching on NFS
  NFS: Display local caching state
  NFS: Store pages from an NFS inode into a local cache
  NFS: Read pages from FS-Cache into an NFS inode
  NFS: nfs_readpage_async() needs to be accessible as a fallback for local caching
  NFS: Add read context retention for FS-Cache to call back with
  NFS: FS-Cache page management
  NFS: Add some new I/O counters for FS-Cache doing things for NFS
  NFS: Invalidate FsCache page flags when cache removed
  NFS: Use local disk inode cache
  NFS: Define and create inode-level cache objects
  NFS: Define and create superblock-level objects
  NFS: Define and create server-level objects
  NFS: Register NFS for caching and retrieve the top-level index
  NFS: Permit local filesystem caching to be enabled for NFS
  NFS: Add FS-Cache option bit and debug bit
  NFS: Add comment banners to some NFS functions
  FS-Cache: Make kAFS use FS-Cache
  CacheFiles: A cache that backs onto a mounted filesystem
  CacheFiles: Export things for CacheFiles
  ...
This commit is contained in:
Linus Torvalds
2009-04-03 10:07:43 -07:00
parent d9b9be024a b797cac748
commit 3cc50ac0db
82 changed files with 15438 additions and 414 deletions
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658
Documentation/filesystems/caching/backend-api.txt Normal file
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==========================
FS-CACHE CACHE BACKEND API
==========================
The FS-Cache system provides an API by which actual caches can be supplied to
FS-Cache for it to then serve out to network filesystems and other interested
parties.
This API is declared in <linux/fscache-cache.h>.
====================================
INITIALISING AND REGISTERING A CACHE
====================================
To start off, a cache definition must be initialised and registered for each
cache the backend wants to make available. For instance, CacheFS does this in
the fill_super() operation on mounting.
The cache definition (struct fscache_cache) should be initialised by calling:
void fscache_init_cache(struct fscache_cache *cache,
struct fscache_cache_ops *ops,
const char *idfmt,
...);
Where:
(*) "cache" is a pointer to the cache definition;
(*) "ops" is a pointer to the table of operations that the backend supports on
this cache; and
(*) "idfmt" is a format and printf-style arguments for constructing a label
for the cache.
The cache should then be registered with FS-Cache by passing a pointer to the
previously initialised cache definition to:
int fscache_add_cache(struct fscache_cache *cache,
struct fscache_object *fsdef,
const char *tagname);
Two extra arguments should also be supplied:
(*) "fsdef" which should point to the object representation for the FS-Cache
master index in this cache. Netfs primary index entries will be created
here. FS-Cache keeps the caller's reference to the index object if
successful and will release it upon withdrawal of the cache.
(*) "tagname" which, if given, should be a text string naming this cache. If
this is NULL, the identifier will be used instead. For CacheFS, the
identifier is set to name the underlying block device and the tag can be
supplied by mount.
This function may return -ENOMEM if it ran out of memory or -EEXIST if the tag
is already in use. 0 will be returned on success.
=====================
UNREGISTERING A CACHE
=====================
A cache can be withdrawn from the system by calling this function with a
pointer to the cache definition:
void fscache_withdraw_cache(struct fscache_cache *cache);
In CacheFS's case, this is called by put_super().
========
SECURITY
========
The cache methods are executed one of two contexts:
(1) that of the userspace process that issued the netfs operation that caused
the cache method to be invoked, or
(2) that of one of the processes in the FS-Cache thread pool.
In either case, this may not be an appropriate context in which to access the
cache.
The calling process's fsuid, fsgid and SELinux security identities may need to
be masqueraded for the duration of the cache driver's access to the cache.
This is left to the cache to handle; FS-Cache makes no effort in this regard.
===================================
CONTROL AND STATISTICS PRESENTATION
===================================
The cache may present data to the outside world through FS-Cache's interfaces
in sysfs and procfs - the former for control and the latter for statistics.
A sysfs directory called /sys/fs/fscache/<cachetag>/ is created if CONFIG_SYSFS
is enabled. This is accessible through the kobject struct fscache_cache::kobj
and is for use by the cache as it sees fit.
========================
RELEVANT DATA STRUCTURES
========================
(*) Index/Data file FS-Cache representation cookie:
struct fscache_cookie {
struct fscache_object_def *def;
struct fscache_netfs *netfs;
void *netfs_data;
...
};
The fields that might be of use to the backend describe the object
definition, the netfs definition and the netfs's data for this cookie.
The object definition contain functions supplied by the netfs for loading
and matching index entries; these are required to provide some of the
cache operations.
(*) In-cache object representation:
struct fscache_object {
int debug_id;
enum {
FSCACHE_OBJECT_RECYCLING,
...
} state;
spinlock_t lock
struct fscache_cache *cache;
struct fscache_cookie *cookie;
...
};
Structures of this type should be allocated by the cache backend and
passed to FS-Cache when requested by the appropriate cache operation. In
the case of CacheFS, they're embedded in CacheFS's internal object
structures.
The debug_id is a simple integer that can be used in debugging messages
that refer to a particular object. In such a case it should be printed
using "OBJ%x" to be consistent with FS-Cache.
Each object contains a pointer to the cookie that represents the object it
is backing. An object should retired when put_object() is called if it is
in state FSCACHE_OBJECT_RECYCLING. The fscache_object struct should be
initialised by calling fscache_object_init(object).
(*) FS-Cache operation record:
struct fscache_operation {
atomic_t usage;
struct fscache_object *object;
unsigned long flags;
#define FSCACHE_OP_EXCLUSIVE
void (*processor)(struct fscache_operation *op);
void (*release)(struct fscache_operation *op);
...
};
FS-Cache has a pool of threads that it uses to give CPU time to the
various asynchronous operations that need to be done as part of driving
the cache. These are represented by the above structure. The processor
method is called to give the op CPU time, and the release method to get
rid of it when its usage count reaches 0.
An operation can be made exclusive upon an object by setting the
appropriate flag before enqueuing it with fscache_enqueue_operation(). If
an operation needs more processing time, it should be enqueued again.
(*) FS-Cache retrieval operation record:
struct fscache_retrieval {
struct fscache_operation op;
struct address_space *mapping;
struct list_head *to_do;
...
};
A structure of this type is allocated by FS-Cache to record retrieval and
allocation requests made by the netfs. This struct is then passed to the
backend to do the operation. The backend may get extra refs to it by
calling fscache_get_retrieval() and refs may be discarded by calling
fscache_put_retrieval().
A retrieval operation can be used by the backend to do retrieval work. To
do this, the retrieval->op.processor method pointer should be set
appropriately by the backend and fscache_enqueue_retrieval() called to
submit it to the thread pool. CacheFiles, for example, uses this to queue
page examination when it detects PG_lock being cleared.
The to_do field is an empty list available for the cache backend to use as
it sees fit.
(*) FS-Cache storage operation record:
struct fscache_storage {
struct fscache_operation op;
pgoff_t store_limit;
...
};
A structure of this type is allocated by FS-Cache to record outstanding
writes to be made. FS-Cache itself enqueues this operation and invokes
the write_page() method on the object at appropriate times to effect
storage.
================
CACHE OPERATIONS
================
The cache backend provides FS-Cache with a table of operations that can be
performed on the denizens of the cache. These are held in a structure of type:
struct fscache_cache_ops
(*) Name of cache provider [mandatory]:
const char *name
This isn't strictly an operation, but should be pointed at a string naming
the backend.
(*) Allocate a new object [mandatory]:
struct fscache_object *(*alloc_object)(struct fscache_cache *cache,
struct fscache_cookie *cookie)
This method is used to allocate a cache object representation to back a
cookie in a particular cache. fscache_object_init() should be called on
the object to initialise it prior to returning.
This function may also be used to parse the index key to be used for
multiple lookup calls to turn it into a more convenient form. FS-Cache
will call the lookup_complete() method to allow the cache to release the
form once lookup is complete or aborted.
(*) Look up and create object [mandatory]:
void (*lookup_object)(struct fscache_object *object)
This method is used to look up an object, given that the object is already
allocated and attached to the cookie. This should instantiate that object
in the cache if it can.
The method should call fscache_object_lookup_negative() as soon as
possible if it determines the object doesn't exist in the cache. If the
object is found to exist and the netfs indicates that it is valid then
fscache_obtained_object() should be called once the object is in a
position to have data stored in it. Similarly, fscache_obtained_object()
should also be called once a non-present object has been created.
If a lookup error occurs, fscache_object_lookup_error() should be called
to abort the lookup of that object.
(*) Release lookup data [mandatory]:
void (*lookup_complete)(struct fscache_object *object)
This method is called to ask the cache to release any resources it was
using to perform a lookup.
(*) Increment object refcount [mandatory]:
struct fscache_object *(*grab_object)(struct fscache_object *object)
This method is called to increment the reference count on an object. It
may fail (for instance if the cache is being withdrawn) by returning NULL.
It should return the object pointer if successful.
(*) Lock/Unlock object [mandatory]:
void (*lock_object)(struct fscache_object *object)
void (*unlock_object)(struct fscache_object *object)
These methods are used to exclusively lock an object. It must be possible
to schedule with the lock held, so a spinlock isn't sufficient.
(*) Pin/Unpin object [optional]:
int (*pin_object)(struct fscache_object *object)
void (*unpin_object)(struct fscache_object *object)
These methods are used to pin an object into the cache. Once pinned an
object cannot be reclaimed to make space. Return -ENOSPC if there's not
enough space in the cache to permit this.
(*) Update object [mandatory]:
int (*update_object)(struct fscache_object *object)
This is called to update the index entry for the specified object. The
new information should be in object->cookie->netfs_data. This can be
obtained by calling object->cookie->def->get_aux()/get_attr().
(*) Discard object [mandatory]:
void (*drop_object)(struct fscache_object *object)
This method is called to indicate that an object has been unbound from its
cookie, and that the cache should release the object's resources and
retire it if it's in state FSCACHE_OBJECT_RECYCLING.
This method should not attempt to release any references held by the
caller. The caller will invoke the put_object() method as appropriate.
(*) Release object reference [mandatory]:
void (*put_object)(struct fscache_object *object)
This method is used to discard a reference to an object. The object may
be freed when all the references to it are released.
(*) Synchronise a cache [mandatory]:
void (*sync)(struct fscache_cache *cache)
This is called to ask the backend to synchronise a cache with its backing
device.
(*) Dissociate a cache [mandatory]:
void (*dissociate_pages)(struct fscache_cache *cache)
This is called to ask a cache to perform any page dissociations as part of
cache withdrawal.
(*) Notification that the attributes on a netfs file changed [mandatory]:
int (*attr_changed)(struct fscache_object *object);
This is called to indicate to the cache that certain attributes on a netfs
file have changed (for example the maximum size a file may reach). The
cache can read these from the netfs by calling the cookie's get_attr()
method.
The cache may use the file size information to reserve space on the cache.
It should also call fscache_set_store_limit() to indicate to FS-Cache the
highest byte it's willing to store for an object.
This method may return -ve if an error occurred or the cache object cannot
be expanded. In such a case, the object will be withdrawn from service.
This operation is run asynchronously from FS-Cache's thread pool, and
storage and retrieval operations from the netfs are excluded during the
execution of this operation.
(*) Reserve cache space for an object's data [optional]:
int (*reserve_space)(struct fscache_object *object, loff_t size);
This is called to request that cache space be reserved to hold the data
for an object and the metadata used to track it. Zero size should be
taken as request to cancel a reservation.
This should return 0 if successful, -ENOSPC if there isn't enough space
available, or -ENOMEM or -EIO on other errors.
The reservation may exceed the current size of the object, thus permitting
future expansion. If the amount of space consumed by an object would
exceed the reservation, it's permitted to refuse requests to allocate
pages, but not required. An object may be pruned down to its reservation
size if larger than that already.
(*) Request page be read from cache [mandatory]:
int (*read_or_alloc_page)(struct fscache_retrieval *op,
struct page *page,
gfp_t gfp)
This is called to attempt to read a netfs page from the cache, or to
reserve a backing block if not. FS-Cache will have done as much checking
as it can before calling, but most of the work belongs to the backend.
If there's no page in the cache, then -ENODATA should be returned if the
backend managed to reserve a backing block; -ENOBUFS or -ENOMEM if it
didn't.
If there is suitable data in the cache, then a read operation should be
queued and 0 returned. When the read finishes, fscache_end_io() should be
called.
The fscache_mark_pages_cached() should be called for the page if any cache
metadata is retained. This will indicate to the netfs that the page needs
explicit uncaching. This operation takes a pagevec, thus allowing several
pages to be marked at once.
The retrieval record pointed to by op should be retained for each page
queued and released when I/O on the page has been formally ended.
fscache_get/put_retrieval() are available for this purpose.
The retrieval record may be used to get CPU time via the FS-Cache thread
pool. If this is desired, the op->op.processor should be set to point to
the appropriate processing routine, and fscache_enqueue_retrieval() should
be called at an appropriate point to request CPU time. For instance, the
retrieval routine could be enqueued upon the completion of a disk read.
The to_do field in the retrieval record is provided to aid in this.
If an I/O error occurs, fscache_io_error() should be called and -ENOBUFS
returned if possible or fscache_end_io() called with a suitable error
code..
(*) Request pages be read from cache [mandatory]:
int (*read_or_alloc_pages)(struct fscache_retrieval *op,
struct list_head *pages,
unsigned *nr_pages,
gfp_t gfp)
This is like the read_or_alloc_page() method, except it is handed a list
of pages instead of one page. Any pages on which a read operation is
started must be added to the page cache for the specified mapping and also
to the LRU. Such pages must also be removed from the pages list and
*nr_pages decremented per page.
If there was an error such as -ENOMEM, then that should be returned; else
if one or more pages couldn't be read or allocated, then -ENOBUFS should
be returned; else if one or more pages couldn't be read, then -ENODATA
should be returned. If all the pages are dispatched then 0 should be
returned.
(*) Request page be allocated in the cache [mandatory]:
int (*allocate_page)(struct fscache_retrieval *op,
struct page *page,
gfp_t gfp)
This is like the read_or_alloc_page() method, except that it shouldn't
read from the cache, even if there's data there that could be retrieved.
It should, however, set up any internal metadata required such that
the write_page() method can write to the cache.
If there's no backing block available, then -ENOBUFS should be returned
(or -ENOMEM if there were other problems). If a block is successfully
allocated, then the netfs page should be marked and 0 returned.
(*) Request pages be allocated in the cache [mandatory]:
int (*allocate_pages)(struct fscache_retrieval *op,
struct list_head *pages,
unsigned *nr_pages,
gfp_t gfp)
This is an multiple page version of the allocate_page() method. pages and
nr_pages should be treated as for the read_or_alloc_pages() method.
(*) Request page be written to cache [mandatory]:
int (*write_page)(struct fscache_storage *op,
struct page *page);
This is called to write from a page on which there was a previously
successful read_or_alloc_page() call or similar. FS-Cache filters out
pages that don't have mappings.
This method is called asynchronously from the FS-Cache thread pool. It is
not required to actually store anything, provided -ENODATA is then
returned to the next read of this page.
If an error occurred, then a negative error code should be returned,
otherwise zero should be returned. FS-Cache will take appropriate action
in response to an error, such as withdrawing this object.
If this method returns success then FS-Cache will inform the netfs
appropriately.
(*) Discard retained per-page metadata [mandatory]:
void (*uncache_page)(struct fscache_object *object, struct page *page)
This is called when a netfs page is being evicted from the pagecache. The
cache backend should tear down any internal representation or tracking it
maintains for this page.
==================
FS-CACHE UTILITIES
==================
FS-Cache provides some utilities that a cache backend may make use of:
(*) Note occurrence of an I/O error in a cache:
void fscache_io_error(struct fscache_cache *cache)
This tells FS-Cache that an I/O error occurred in the cache. After this
has been called, only resource dissociation operations (object and page
release) will be passed from the netfs to the cache backend for the
specified cache.
This does not actually withdraw the cache. That must be done separately.
(*) Invoke the retrieval I/O completion function:
void fscache_end_io(struct fscache_retrieval *op, struct page *page,
int error);
This is called to note the end of an attempt to retrieve a page. The
error value should be 0 if successful and an error otherwise.
(*) Set highest store limit:
void fscache_set_store_limit(struct fscache_object *object,
loff_t i_size);
This sets the limit FS-Cache imposes on the highest byte it's willing to
try and store for a netfs. Any page over this limit is automatically
rejected by fscache_read_alloc_page() and co with -ENOBUFS.
(*) Mark pages as being cached:
void fscache_mark_pages_cached(struct fscache_retrieval *op,
struct pagevec *pagevec);
This marks a set of pages as being cached. After this has been called,
the netfs must call fscache_uncache_page() to unmark the pages.
(*) Perform coherency check on an object:
enum fscache_checkaux fscache_check_aux(struct fscache_object *object,
const void *data,
uint16_t datalen);
This asks the netfs to perform a coherency check on an object that has
just been looked up. The cookie attached to the object will determine the
netfs to use. data and datalen should specify where the auxiliary data
retrieved from the cache can be found.
One of three values will be returned:
(*) FSCACHE_CHECKAUX_OKAY
The coherency data indicates the object is valid as is.
(*) FSCACHE_CHECKAUX_NEEDS_UPDATE
The coherency data needs updating, but otherwise the object is
valid.
(*) FSCACHE_CHECKAUX_OBSOLETE
The coherency data indicates that the object is obsolete and should
be discarded.
(*) Initialise a freshly allocated object:
void fscache_object_init(struct fscache_object *object);
This initialises all the fields in an object representation.
(*) Indicate the destruction of an object:
void fscache_object_destroyed(struct fscache_cache *cache);
This must be called to inform FS-Cache that an object that belonged to a
cache has been destroyed and deallocated. This will allow continuation
of the cache withdrawal process when it is stopped pending destruction of
all the objects.
(*) Indicate negative lookup on an object:
void fscache_object_lookup_negative(struct fscache_object *object);
This is called to indicate to FS-Cache that a lookup process for an object
found a negative result.
This changes the state of an object to permit reads pending on lookup
completion to go off and start fetching data from the netfs server as it's
known at this point that there can't be any data in the cache.
This may be called multiple times on an object. Only the first call is
significant - all subsequent calls are ignored.
(*) Indicate an object has been obtained:
void fscache_obtained_object(struct fscache_object *object);
This is called to indicate to FS-Cache that a lookup process for an object
produced a positive result, or that an object was created. This should
only be called once for any particular object.
This changes the state of an object to indicate:
(1) if no call to fscache_object_lookup_negative() has been made on
this object, that there may be data available, and that reads can
now go and look for it; and
(2) that writes may now proceed against this object.
(*) Indicate that object lookup failed:
void fscache_object_lookup_error(struct fscache_object *object);
This marks an object as having encountered a fatal error (usually EIO)
and causes it to move into a state whereby it will be withdrawn as soon
as possible.
(*) Get and release references on a retrieval record:
void fscache_get_retrieval(struct fscache_retrieval *op);
void fscache_put_retrieval(struct fscache_retrieval *op);
These two functions are used to retain a retrieval record whilst doing
asynchronous data retrieval and block allocation.
(*) Enqueue a retrieval record for processing.
void fscache_enqueue_retrieval(struct fscache_retrieval *op);
This enqueues a retrieval record for processing by the FS-Cache thread
pool. One of the threads in the pool will invoke the retrieval record's
op->op.processor callback function. This function may be called from
within the callback function.
(*) List of object state names:
const char *fscache_object_states[];
For debugging purposes, this may be used to turn the state that an object
is in into a text string for display purposes.

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===============================================
CacheFiles: CACHE ON ALREADY MOUNTED FILESYSTEM
===============================================
Contents:
(*) Overview.
(*) Requirements.
(*) Configuration.
(*) Starting the cache.
(*) Things to avoid.
(*) Cache culling.
(*) Cache structure.
(*) Security model and SELinux.
(*) A note on security.
(*) Statistical information.
(*) Debugging.
========
OVERVIEW
========
CacheFiles is a caching backend that's meant to use as a cache a directory on
an already mounted filesystem of a local type (such as Ext3).
CacheFiles uses a userspace daemon to do some of the cache management - such as
reaping stale nodes and culling. This is called cachefilesd and lives in
/sbin.
The filesystem and data integrity of the cache are only as good as those of the
filesystem providing the backing services. Note that CacheFiles does not
attempt to journal anything since the journalling interfaces of the various
filesystems are very specific in nature.
CacheFiles creates a misc character device - "/dev/cachefiles" - that is used
to communication with the daemon. Only one thing may have this open at once,
and whilst it is open, a cache is at least partially in existence. The daemon
opens this and sends commands down it to control the cache.
CacheFiles is currently limited to a single cache.
CacheFiles attempts to maintain at least a certain percentage of free space on
the filesystem, shrinking the cache by culling the objects it contains to make
space if necessary - see the "Cache Culling" section. This means it can be
placed on the same medium as a live set of data, and will expand to make use of
spare space and automatically contract when the set of data requires more
space.
============
REQUIREMENTS
============
The use of CacheFiles and its daemon requires the following features to be
available in the system and in the cache filesystem:
- dnotify.
- extended attributes (xattrs).
- openat() and friends.
- bmap() support on files in the filesystem (FIBMAP ioctl).
- The use of bmap() to detect a partial page at the end of the file.
It is strongly recommended that the "dir_index" option is enabled on Ext3
filesystems being used as a cache.
=============
CONFIGURATION
=============
The cache is configured by a script in /etc/cachefilesd.conf. These commands
set up cache ready for use. The following script commands are available:
(*) brun <N>%
(*) bcull <N>%
(*) bstop <N>%
(*) frun <N>%
(*) fcull <N>%
(*) fstop <N>%
Configure the culling limits. Optional. See the section on culling
The defaults are 7% (run), 5% (cull) and 1% (stop) respectively.
The commands beginning with a 'b' are file space (block) limits, those
beginning with an 'f' are file count limits.
(*) dir <path>
Specify the directory containing the root of the cache. Mandatory.
(*) tag <name>
Specify a tag to FS-Cache to use in distinguishing multiple caches.
Optional. The default is "CacheFiles".
(*) debug <mask>
Specify a numeric bitmask to control debugging in the kernel module.
Optional. The default is zero (all off). The following values can be
OR'd into the mask to collect various information:
1 Turn on trace of function entry (_enter() macros)
2 Turn on trace of function exit (_leave() macros)
4 Turn on trace of internal debug points (_debug())
This mask can also be set through sysfs, eg:
echo 5 >/sys/modules/cachefiles/parameters/debug
==================
STARTING THE CACHE
==================
The cache is started by running the daemon. The daemon opens the cache device,
configures the cache and tells it to begin caching. At that point the cache
binds to fscache and the cache becomes live.
The daemon is run as follows:
/sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>]
The flags are:
(*) -d
Increase the debugging level. This can be specified multiple times and
is cumulative with itself.
(*) -s
Send messages to stderr instead of syslog.
(*) -n
Don't daemonise and go into background.
(*) -f <configfile>
Use an alternative configuration file rather than the default one.
===============
THINGS TO AVOID
===============
Do not mount other things within the cache as this will cause problems. The
kernel module contains its own very cut-down path walking facility that ignores
mountpoints, but the daemon can't avoid them.
Do not create, rename or unlink files and directories in the cache whilst the
cache is active, as this may cause the state to become uncertain.
Renaming files in the cache might make objects appear to be other objects (the
filename is part of the lookup key).
Do not change or remove the extended attributes attached to cache files by the
cache as this will cause the cache state management to get confused.
Do not create files or directories in the cache, lest the cache get confused or
serve incorrect data.
Do not chmod files in the cache. The module creates things with minimal
permissions to prevent random users being able to access them directly.
=============
CACHE CULLING
=============
The cache may need culling occasionally to make space. This involves
discarding objects from the cache that have been used less recently than
anything else. Culling is based on the access time of data objects. Empty
directories are culled if not in use.
Cache culling is done on the basis of the percentage of blocks and the
percentage of files available in the underlying filesystem. There are six
"limits":
(*) brun
(*) frun
If the amount of free space and the number of available files in the cache
rises above both these limits, then culling is turned off.
(*) bcull
(*) fcull
If the amount of available space or the number of available files in the
cache falls below either of these limits, then culling is started.
(*) bstop
(*) fstop
If the amount of available space or the number of available files in the
cache falls below either of these limits, then no further allocation of
disk space or files is permitted until culling has raised things above
these limits again.
These must be configured thusly:
0 <= bstop < bcull < brun < 100
0 <= fstop < fcull < frun < 100
Note that these are percentages of available space and available files, and do
_not_ appear as 100 minus the percentage displayed by the "df" program.
The userspace daemon scans the cache to build up a table of cullable objects.
These are then culled in least recently used order. A new scan of the cache is
started as soon as space is made in the table. Objects will be skipped if
their atimes have changed or if the kernel module says it is still using them.
===============
CACHE STRUCTURE
===============
The CacheFiles module will create two directories in the directory it was
given:
(*) cache/
(*) graveyard/
The active cache objects all reside in the first directory. The CacheFiles
kernel module moves any retired or culled objects that it can't simply unlink
to the graveyard from which the daemon will actually delete them.
The daemon uses dnotify to monitor the graveyard directory, and will delete
anything that appears therein.
The module represents index objects as directories with the filename "I..." or
"J...". Note that the "cache/" directory is itself a special index.
Data objects are represented as files if they have no children, or directories
if they do. Their filenames all begin "D..." or "E...". If represented as a
directory, data objects will have a file in the directory called "data" that
actually holds the data.
Special objects are similar to data objects, except their filenames begin
"S..." or "T...".
If an object has children, then it will be represented as a directory.
Immediately in the representative directory are a collection of directories
named for hash values of the child object keys with an '@' prepended. Into
this directory, if possible, will be placed the representations of the child
objects:
INDEX INDEX INDEX DATA FILES
========= ========== ================================= ================
cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400
cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry
cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry
cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry
If the key is so long that it exceeds NAME_MAX with the decorations added on to
it, then it will be cut into pieces, the first few of which will be used to
make a nest of directories, and the last one of which will be the objects
inside the last directory. The names of the intermediate directories will have
'+' prepended:
J1223/@23/+xy...z/+kl...m/Epqr
Note that keys are raw data, and not only may they exceed NAME_MAX in size,
they may also contain things like '/' and NUL characters, and so they may not
be suitable for turning directly into a filename.
To handle this, CacheFiles will use a suitably printable filename directly and
"base-64" encode ones that aren't directly suitable. The two versions of
object filenames indicate the encoding:
OBJECT TYPE PRINTABLE ENCODED
=============== =============== ===============
Index "I..." "J..."
Data "D..." "E..."
Special "S..." "T..."
Intermediate directories are always "@" or "+" as appropriate.
Each object in the cache has an extended attribute label that holds the object
type ID (required to distinguish special objects) and the auxiliary data from
the netfs. The latter is used to detect stale objects in the cache and update
or retire them.
Note that CacheFiles will erase from the cache any file it doesn't recognise or
any file of an incorrect type (such as a FIFO file or a device file).
==========================
SECURITY MODEL AND SELINUX
==========================
CacheFiles is implemented to deal properly with the LSM security features of
the Linux kernel and the SELinux facility.
One of the problems that CacheFiles faces is that it is generally acting on
behalf of a process, and running in that process's context, and that includes a
security context that is not appropriate for accessing the cache - either
because the files in the cache are inaccessible to that process, or because if
the process creates a file in the cache, that file may be inaccessible to other
processes.
The way CacheFiles works is to temporarily change the security context (fsuid,
fsgid and actor security label) that the process acts as - without changing the
security context of the process when it the target of an operation performed by
some other process (so signalling and suchlike still work correctly).
When the CacheFiles module is asked to bind to its cache, it:
(1) Finds the security label attached to the root cache directory and uses
that as the security label with which it will create files. By default,
this is:
cachefiles_var_t
(2) Finds the security label of the process which issued the bind request
(presumed to be the cachefilesd daemon), which by default will be:
cachefilesd_t
and asks LSM to supply a security ID as which it should act given the
daemon's label. By default, this will be:
cachefiles_kernel_t
SELinux transitions the daemon's security ID to the module's security ID
based on a rule of this form in the policy.
type_transition <daemon's-ID> kernel_t : process <module's-ID>;
For instance:
type_transition cachefilesd_t kernel_t : process cachefiles_kernel_t;
The module's security ID gives it permission to create, move and remove files
and directories in the cache, to find and access directories and files in the
cache, to set and access extended attributes on cache objects, and to read and
write files in the cache.
The daemon's security ID gives it only a very restricted set of permissions: it
may scan directories, stat files and erase files and directories. It may
not read or write files in the cache, and so it is precluded from accessing the
data cached therein; nor is it permitted to create new files in the cache.
There are policy source files available in:
http://people.redhat.com/~dhowells/fscache/cachefilesd-0.8.tar.bz2
and later versions. In that tarball, see the files:
cachefilesd.te
cachefilesd.fc
cachefilesd.if
They are built and installed directly by the RPM.
If a non-RPM based system is being used, then copy the above files to their own
directory and run:
make -f /usr/share/selinux/devel/Makefile
semodule -i cachefilesd.pp
You will need checkpolicy and selinux-policy-devel installed prior to the
build.
By default, the cache is located in /var/fscache, but if it is desirable that
it should be elsewhere, than either the above policy files must be altered, or
an auxiliary policy must be installed to label the alternate location of the
cache.
For instructions on how to add an auxiliary policy to enable the cache to be
located elsewhere when SELinux is in enforcing mode, please see:
/usr/share/doc/cachefilesd-*/move-cache.txt
When the cachefilesd rpm is installed; alternatively, the document can be found
in the sources.
==================
A NOTE ON SECURITY
==================
CacheFiles makes use of the split security in the task_struct. It allocates
its own task_security structure, and redirects current->act_as to point to it
when it acts on behalf of another process, in that process's context.
The reason it does this is that it calls vfs_mkdir() and suchlike rather than
bypassing security and calling inode ops directly. Therefore the VFS and LSM
may deny the CacheFiles access to the cache data because under some
circumstances the caching code is running in the security context of whatever
process issued the original syscall on the netfs.
Furthermore, should CacheFiles create a file or directory, the security
parameters with that object is created (UID, GID, security label) would be
derived from that process that issued the system call, thus potentially
preventing other processes from accessing the cache - including CacheFiles's
cache management daemon (cachefilesd).
What is required is to temporarily override the security of the process that
issued the system call. We can't, however, just do an in-place change of the
security data as that affects the process as an object, not just as a subject.
This means it may lose signals or ptrace events for example, and affects what
the process looks like in /proc.
So CacheFiles makes use of a logical split in the security between the
objective security (task->sec) and the subjective security (task->act_as). The
objective security holds the intrinsic security properties of a process and is
never overridden. This is what appears in /proc, and is what is used when a
process is the target of an operation by some other process (SIGKILL for
example).
The subjective security holds the active security properties of a process, and
may be overridden. This is not seen externally, and is used whan a process
acts upon another object, for example SIGKILLing another process or opening a
file.
LSM hooks exist that allow SELinux (or Smack or whatever) to reject a request
for CacheFiles to run in a context of a specific security label, or to create
files and directories with another security label.
=======================
STATISTICAL INFORMATION
=======================
If FS-Cache is compiled with the following option enabled:
CONFIG_CACHEFILES_HISTOGRAM=y
then it will gather certain statistics and display them through a proc file.
(*) /proc/fs/cachefiles/histogram
cat /proc/fs/cachefiles/histogram
JIFS SECS LOOKUPS MKDIRS CREATES
===== ===== ========= ========= =========
This shows the breakdown of the number of times each amount of time
between 0 jiffies and HZ-1 jiffies a variety of tasks took to run. The
columns are as follows:
COLUMN TIME MEASUREMENT
======= =======================================================
LOOKUPS Length of time to perform a lookup on the backing fs
MKDIRS Length of time to perform a mkdir on the backing fs
CREATES Length of time to perform a create on the backing fs
Each row shows the number of events that took a particular range of times.
Each step is 1 jiffy in size. The JIFS column indicates the particular
jiffy range covered, and the SECS field the equivalent number of seconds.
=========
DEBUGGING
=========
If CONFIG_CACHEFILES_DEBUG is enabled, the CacheFiles facility can have runtime
debugging enabled by adjusting the value in:
/sys/module/cachefiles/parameters/debug
This is a bitmask of debugging streams to enable:
BIT VALUE STREAM POINT
======= ======= =============================== =======================
0 1 General Function entry trace
1 2 Function exit trace
2 4 General
The appropriate set of values should be OR'd together and the result written to
the control file. For example:
echo $((1|4|8)) >/sys/module/cachefiles/parameters/debug
will turn on all function entry debugging.

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==========================
General Filesystem Caching
==========================
========
OVERVIEW
========
This facility is a general purpose cache for network filesystems, though it
could be used for caching other things such as ISO9660 filesystems too.
FS-Cache mediates between cache backends (such as CacheFS) and network
filesystems:
+---------+
| | +--------------+
| NFS |--+ | |
| | | +-->| CacheFS |
+---------+ | +----------+ | | /dev/hda5 |
| | | | +--------------+
+---------+ +-->| | |
| | | |--+
| AFS |----->| FS-Cache |
| | | |--+
+---------+ +-->| | |
| | | | +--------------+
+---------+ | +----------+ | | |
| | | +-->| CacheFiles |
| ISOFS |--+ | /var/cache |
| | +--------------+
+---------+
Or to look at it another way, FS-Cache is a module that provides a caching
facility to a network filesystem such that the cache is transparent to the
user:
+---------+
| |
| Server |
| |
+---------+
| NETWORK
~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
| +----------+
V | |
+---------+ | |
| | | |
| NFS |----->| FS-Cache |
| | | |--+
+---------+ | | | +--------------+ +--------------+
| | | | | | | |
V +----------+ +-->| CacheFiles |-->| Ext3 |
+---------+ | /var/cache | | /dev/sda6 |
| | +--------------+ +--------------+
| VFS | ^ ^
| | | |
+---------+ +--------------+ |
| KERNEL SPACE | |
~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|~~~~~~|~~~~
| USER SPACE | |
V | |
+---------+ +--------------+
| | | |
| Process | | cachefilesd |
| | | |
+---------+ +--------------+
FS-Cache does not follow the idea of completely loading every netfs file
opened in its entirety into a cache before permitting it to be accessed and
then serving the pages out of that cache rather than the netfs inode because:
(1) It must be practical to operate without a cache.
(2) The size of any accessible file must not be limited to the size of the
cache.
(3) The combined size of all opened files (this includes mapped libraries)
must not be limited to the size of the cache.
(4) The user should not be forced to download an entire file just to do a
one-off access of a small portion of it (such as might be done with the
"file" program).
It instead serves the cache out in PAGE_SIZE chunks as and when requested by
the netfs('s) using it.
FS-Cache provides the following facilities:
(1) More than one cache can be used at once. Caches can be selected
explicitly by use of tags.
(2) Caches can be added / removed at any time.
(3) The netfs is provided with an interface that allows either party to
withdraw caching facilities from a file (required for (2)).
(4) The interface to the netfs returns as few errors as possible, preferring
rather to let the netfs remain oblivious.
(5) Cookies are used to represent indices, files and other objects to the
netfs. The simplest cookie is just a NULL pointer - indicating nothing
cached there.
(6) The netfs is allowed to propose - dynamically - any index hierarchy it
desires, though it must be aware that the index search function is
recursive, stack space is limited, and indices can only be children of
indices.
(7) Data I/O is done direct to and from the netfs's pages. The netfs
indicates that page A is at index B of the data-file represented by cookie
C, and that it should be read or written. The cache backend may or may
not start I/O on that page, but if it does, a netfs callback will be
invoked to indicate completion. The I/O may be either synchronous or
asynchronous.
(8) Cookies can be "retired" upon release. At this point FS-Cache will mark
them as obsolete and the index hierarchy rooted at that point will get
recycled.
(9) The netfs provides a "match" function for index searches. In addition to
saying whether a match was made or not, this can also specify that an
entry should be updated or deleted.
(10) As much as possible is done asynchronously.
FS-Cache maintains a virtual indexing tree in which all indices, files, objects
and pages are kept. Bits of this tree may actually reside in one or more
caches.
FSDEF
|
+------------------------------------+
| |
NFS AFS
| |
+--------------------------+ +-----------+
| | | |
homedir mirror afs.org redhat.com
| | |
+------------+ +---------------+ +----------+
| | | | | |
00001 00002 00007 00125 vol00001 vol00002
| | | | |
+---+---+ +-----+ +---+ +------+------+ +-----+----+
| | | | | | | | | | | | |
PG0 PG1 PG2 PG0 XATTR PG0 PG1 DIRENT DIRENT DIRENT R/W R/O Bak
| |
PG0 +-------+
| |
00001 00003
|
+---+---+
| | |
PG0 PG1 PG2
In the example above, you can see two netfs's being backed: NFS and AFS. These
have different index hierarchies:
(*) The NFS primary index contains per-server indices. Each server index is
indexed by NFS file handles to get data file objects. Each data file
objects can have an array of pages, but may also have further child
objects, such as extended attributes and directory entries. Extended
attribute objects themselves have page-array contents.
(*) The AFS primary index contains per-cell indices. Each cell index contains
per-logical-volume indices. Each of volume index contains up to three
indices for the read-write, read-only and backup mirrors of those volumes.
Each of these contains vnode data file objects, each of which contains an
array of pages.
The very top index is the FS-Cache master index in which individual netfs's
have entries.
Any index object may reside in more than one cache, provided it only has index
children. Any index with non-index object children will be assumed to only
reside in one cache.
The netfs API to FS-Cache can be found in:
Documentation/filesystems/caching/netfs-api.txt
The cache backend API to FS-Cache can be found in:
Documentation/filesystems/caching/backend-api.txt
A description of the internal representations and object state machine can be
found in:
Documentation/filesystems/caching/object.txt
=======================
STATISTICAL INFORMATION
=======================
If FS-Cache is compiled with the following options enabled:
CONFIG_FSCACHE_STATS=y
CONFIG_FSCACHE_HISTOGRAM=y
then it will gather certain statistics and display them through a number of
proc files.
(*) /proc/fs/fscache/stats
This shows counts of a number of events that can happen in FS-Cache:
CLASS EVENT MEANING
======= ======= =======================================================
Cookies idx=N Number of index cookies allocated
dat=N Number of data storage cookies allocated
spc=N Number of special cookies allocated
Objects alc=N Number of objects allocated
nal=N Number of object allocation failures
avl=N Number of objects that reached the available state
ded=N Number of objects that reached the dead state
ChkAux non=N Number of objects that didn't have a coherency check
ok=N Number of objects that passed a coherency check
upd=N Number of objects that needed a coherency data update
obs=N Number of objects that were declared obsolete
Pages mrk=N Number of pages marked as being cached
unc=N Number of uncache page requests seen
Acquire n=N Number of acquire cookie requests seen
nul=N Number of acq reqs given a NULL parent
noc=N Number of acq reqs rejected due to no cache available
ok=N Number of acq reqs succeeded
nbf=N Number of acq reqs rejected due to error
oom=N Number of acq reqs failed on ENOMEM
Lookups n=N Number of lookup calls made on cache backends
neg=N Number of negative lookups made
pos=N Number of positive lookups made
crt=N Number of objects created by lookup
Updates n=N Number of update cookie requests seen
nul=N Number of upd reqs given a NULL parent
run=N Number of upd reqs granted CPU time
Relinqs n=N Number of relinquish cookie requests seen
nul=N Number of rlq reqs given a NULL parent
wcr=N Number of rlq reqs waited on completion of creation
AttrChg n=N Number of attribute changed requests seen
ok=N Number of attr changed requests queued
nbf=N Number of attr changed rejected -ENOBUFS
oom=N Number of attr changed failed -ENOMEM
run=N Number of attr changed ops given CPU time
Allocs n=N Number of allocation requests seen
ok=N Number of successful alloc reqs
wt=N Number of alloc reqs that waited on lookup completion
nbf=N Number of alloc reqs rejected -ENOBUFS
ops=N Number of alloc reqs submitted
owt=N Number of alloc reqs waited for CPU time
Retrvls n=N Number of retrieval (read) requests seen
ok=N Number of successful retr reqs
wt=N Number of retr reqs that waited on lookup completion
nod=N Number of retr reqs returned -ENODATA
nbf=N Number of retr reqs rejected -ENOBUFS
int=N Number of retr reqs aborted -ERESTARTSYS
oom=N Number of retr reqs failed -ENOMEM
ops=N Number of retr reqs submitted
owt=N Number of retr reqs waited for CPU time
Stores n=N Number of storage (write) requests seen
ok=N Number of successful store reqs
agn=N Number of store reqs on a page already pending storage
nbf=N Number of store reqs rejected -ENOBUFS
oom=N Number of store reqs failed -ENOMEM
ops=N Number of store reqs submitted
run=N Number of store reqs granted CPU time
Ops pend=N Number of times async ops added to pending queues
run=N Number of times async ops given CPU time
enq=N Number of times async ops queued for processing
dfr=N Number of async ops queued for deferred release
rel=N Number of async ops released
gc=N Number of deferred-release async ops garbage collected
(*) /proc/fs/fscache/histogram
cat /proc/fs/fscache/histogram
JIFS SECS OBJ INST OP RUNS OBJ RUNS RETRV DLY RETRIEVLS
===== ===== ========= ========= ========= ========= =========
This shows the breakdown of the number of times each amount of time
between 0 jiffies and HZ-1 jiffies a variety of tasks took to run. The
columns are as follows:
COLUMN TIME MEASUREMENT
======= =======================================================
OBJ INST Length of time to instantiate an object
OP RUNS Length of time a call to process an operation took
OBJ RUNS Length of time a call to process an object event took
RETRV DLY Time between an requesting a read and lookup completing
RETRIEVLS Time between beginning and end of a retrieval
Each row shows the number of events that took a particular range of times.
Each step is 1 jiffy in size. The JIFS column indicates the particular
jiffy range covered, and the SECS field the equivalent number of seconds.
=========
DEBUGGING
=========
If CONFIG_FSCACHE_DEBUG is enabled, the FS-Cache facility can have runtime
debugging enabled by adjusting the value in:
/sys/module/fscache/parameters/debug
This is a bitmask of debugging streams to enable:
BIT VALUE STREAM POINT
======= ======= =============================== =======================
0 1 Cache management Function entry trace
1 2 Function exit trace
2 4 General
3 8 Cookie management Function entry trace
4 16 Function exit trace
5 32 General
6 64 Page handling Function entry trace
7 128 Function exit trace
8 256 General
9 512 Operation management Function entry trace
10 1024 Function exit trace
11 2048 General
The appropriate set of values should be OR'd together and the result written to
the control file. For example:
echo $((1|8|64)) >/sys/module/fscache/parameters/debug
will turn on all function entry debugging.

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===============================
FS-CACHE NETWORK FILESYSTEM API
===============================
There's an API by which a network filesystem can make use of the FS-Cache
facilities. This is based around a number of principles:
(1) Caches can store a number of different object types. There are two main
object types: indices and files. The first is a special type used by
FS-Cache to make finding objects faster and to make retiring of groups of
objects easier.
(2) Every index, file or other object is represented by a cookie. This cookie
may or may not have anything associated with it, but the netfs doesn't
need to care.
(3) Barring the top-level index (one entry per cached netfs), the index
hierarchy for each netfs is structured according the whim of the netfs.
This API is declared in <linux/fscache.h>.
This document contains the following sections:
(1) Network filesystem definition
(2) Index definition
(3) Object definition
(4) Network filesystem (un)registration
(5) Cache tag lookup
(6) Index registration
(7) Data file registration
(8) Miscellaneous object registration
(9) Setting the data file size
(10) Page alloc/read/write
(11) Page uncaching
(12) Index and data file update
(13) Miscellaneous cookie operations
(14) Cookie unregistration
(15) Index and data file invalidation
(16) FS-Cache specific page flags.
=============================
NETWORK FILESYSTEM DEFINITION
=============================
FS-Cache needs a description of the network filesystem. This is specified
using a record of the following structure:
struct fscache_netfs {
uint32_t version;
const char *name;
struct fscache_cookie *primary_index;
...
};
This first two fields should be filled in before registration, and the third
will be filled in by the registration function; any other fields should just be
ignored and are for internal use only.
The fields are:
(1) The name of the netfs (used as the key in the toplevel index).
(2) The version of the netfs (if the name matches but the version doesn't, the
entire in-cache hierarchy for this netfs will be scrapped and begun
afresh).
(3) The cookie representing the primary index will be allocated according to
another parameter passed into the registration function.
For example, kAFS (linux/fs/afs/) uses the following definitions to describe
itself:
struct fscache_netfs afs_cache_netfs = {
.version = 0,
.name = "afs",
};
================
INDEX DEFINITION
================
Indices are used for two purposes:
(1) To aid the finding of a file based on a series of keys (such as AFS's
"cell", "volume ID", "vnode ID").
(2) To make it easier to discard a subset of all the files cached based around
a particular key - for instance to mirror the removal of an AFS volume.
However, since it's unlikely that any two netfs's are going to want to define
their index hierarchies in quite the same way, FS-Cache tries to impose as few
restraints as possible on how an index is structured and where it is placed in
the tree. The netfs can even mix indices and data files at the same level, but
it's not recommended.
Each index entry consists of a key of indeterminate length plus some auxilliary
data, also of indeterminate length.
There are some limits on indices:
(1) Any index containing non-index objects should be restricted to a single
cache. Any such objects created within an index will be created in the
first cache only. The cache in which an index is created can be
controlled by cache tags (see below).
(2) The entry data must be atomically journallable, so it is limited to about
400 bytes at present. At least 400 bytes will be available.
(3) The depth of the index tree should be judged with care as the search
function is recursive. Too many layers will run the kernel out of stack.
=================
OBJECT DEFINITION
=================
To define an object, a structure of the following type should be filled out:
struct fscache_cookie_def
{
uint8_t name[16];
uint8_t type;
struct fscache_cache_tag *(*select_cache)(
const void *parent_netfs_data,
const void *cookie_netfs_data);
uint16_t (*get_key)(const void *cookie_netfs_data,
void *buffer,
uint16_t bufmax);
void (*get_attr)(const void *cookie_netfs_data,
uint64_t *size);
uint16_t (*get_aux)(const void *cookie_netfs_data,
void *buffer,
uint16_t bufmax);
enum fscache_checkaux (*check_aux)(void *cookie_netfs_data,
const void *data,
uint16_t datalen);
void (*get_context)(void *cookie_netfs_data, void *context);
void (*put_context)(void *cookie_netfs_data, void *context);
void (*mark_pages_cached)(void *cookie_netfs_data,
struct address_space *mapping,
struct pagevec *cached_pvec);
void (*now_uncached)(void *cookie_netfs_data);
};
This has the following fields:
(1) The type of the object [mandatory].
This is one of the following values:
(*) FSCACHE_COOKIE_TYPE_INDEX
This defines an index, which is a special FS-Cache type.
(*) FSCACHE_COOKIE_TYPE_DATAFILE
This defines an ordinary data file.
(*) Any other value between 2 and 255
This defines an extraordinary object such as an XATTR.
(2) The name of the object type (NUL terminated unless all 16 chars are used)
[optional].
(3) A function to select the cache in which to store an index [optional].
This function is invoked when an index needs to be instantiated in a cache
during the instantiation of a non-index object. Only the immediate index
parent for the non-index object will be queried. Any indices above that
in the hierarchy may be stored in multiple caches. This function does not
need to be supplied for any non-index object or any index that will only
have index children.
If this function is not supplied or if it returns NULL then the first
cache in the parent's list will be chosed, or failing that, the first
cache in the master list.
(4) A function to retrieve an object's key from the netfs [mandatory].
This function will be called with the netfs data that was passed to the
cookie acquisition function and the maximum length of key data that it may
provide. It should write the required key data into the given buffer and
return the quantity it wrote.
(5) A function to retrieve attribute data from the netfs [optional].
This function will be called with the netfs data that was passed to the
cookie acquisition function. It should return the size of the file if
this is a data file. The size may be used to govern how much cache must
be reserved for this file in the cache.
If the function is absent, a file size of 0 is assumed.
(6) A function to retrieve auxilliary data from the netfs [optional].
This function will be called with the netfs data that was passed to the
cookie acquisition function and the maximum length of auxilliary data that
it may provide. It should write the auxilliary data into the given buffer
and return the quantity it wrote.
If this function is absent, the auxilliary data length will be set to 0.
The length of the auxilliary data buffer may be dependent on the key
length. A netfs mustn't rely on being able to provide more than 400 bytes
for both.
(7) A function to check the auxilliary data [optional].
This function will be called to check that a match found in the cache for
this object is valid. For instance with AFS it could check the auxilliary
data against the data version number returned by the server to determine
whether the index entry in a cache is still valid.
If this function is absent, it will be assumed that matching objects in a
cache are always valid.
If present, the function should return one of the following values:
(*) FSCACHE_CHECKAUX_OKAY - the entry is okay as is
(*) FSCACHE_CHECKAUX_NEEDS_UPDATE - the entry requires update
(*) FSCACHE_CHECKAUX_OBSOLETE - the entry should be deleted
This function can also be used to extract data from the auxilliary data in
the cache and copy it into the netfs's structures.
(8) A pair of functions to manage contexts for the completion callback
[optional].
The cache read/write functions are passed a context which is then passed
to the I/O completion callback function. To ensure this context remains
valid until after the I/O completion is called, two functions may be
provided: one to get an extra reference on the context, and one to drop a
reference to it.
If the context is not used or is a type of object that won't go out of
scope, then these functions are not required. These functions are not
required for indices as indices may not contain data. These functions may
be called in interrupt context and so may not sleep.
(9) A function to mark a page as retaining cache metadata [optional].
This is called by the cache to indicate that it is retaining in-memory
information for this page and that the netfs should uncache the page when
it has finished. This does not indicate whether there's data on the disk
or not. Note that several pages at once may be presented for marking.
The PG_fscache bit is set on the pages before this function would be
called, so the function need not be provided if this is sufficient.
This function is not required for indices as they're not permitted data.
(10) A function to unmark all the pages retaining cache metadata [mandatory].
This is called by FS-Cache to indicate that a backing store is being
unbound from a cookie and that all the marks on the pages should be
cleared to prevent confusion. Note that the cache will have torn down all
its tracking information so that the pages don't need to be explicitly
uncached.
This function is not required for indices as they're not permitted data.
===================================
NETWORK FILESYSTEM (UN)REGISTRATION
===================================
The first step is to declare the network filesystem to the cache. This also
involves specifying the layout of the primary index (for AFS, this would be the
"cell" level).
The registration function is:
int fscache_register_netfs(struct fscache_netfs *netfs);
It just takes a pointer to the netfs definition. It returns 0 or an error as
appropriate.
For kAFS, registration is done as follows:
ret = fscache_register_netfs(&afs_cache_netfs);
The last step is, of course, unregistration:
void fscache_unregister_netfs(struct fscache_netfs *netfs);
================
CACHE TAG LOOKUP
================
FS-Cache permits the use of more than one cache. To permit particular index
subtrees to be bound to particular caches, the second step is to look up cache
representation tags. This step is optional; it can be left entirely up to
FS-Cache as to which cache should be used. The problem with doing that is that
FS-Cache will always pick the first cache that was registered.
To get the representation for a named tag:
struct fscache_cache_tag *fscache_lookup_cache_tag(const char *name);
This takes a text string as the name and returns a representation of a tag. It
will never return an error. It may return a dummy tag, however, if it runs out
of memory; this will inhibit caching with this tag.
Any representation so obtained must be released by passing it to this function:
void fscache_release_cache_tag(struct fscache_cache_tag *tag);
The tag will be retrieved by FS-Cache when it calls the object definition
operation select_cache().
==================
INDEX REGISTRATION
==================
The third step is to inform FS-Cache about part of an index hierarchy that can
be used to locate files. This is done by requesting a cookie for each index in
the path to the file:
struct fscache_cookie *
fscache_acquire_cookie(struct fscache_cookie *parent,
const struct fscache_object_def *def,
void *netfs_data);
This function creates an index entry in the index represented by parent,
filling in the index entry by calling the operations pointed to by def.
Note that this function never returns an error - all errors are handled
internally. It may, however, return NULL to indicate no cookie. It is quite
acceptable to pass this token back to this function as the parent to another
acquisition (or even to the relinquish cookie, read page and write page
functions - see below).
Note also that no indices are actually created in a cache until a non-index
object needs to be created somewhere down the hierarchy. Furthermore, an index
may be created in several different caches independently at different times.
This is all handled transparently, and the netfs doesn't see any of it.
For example, with AFS, a cell would be added to the primary index. This index
entry would have a dependent inode containing a volume location index for the
volume mappings within this cell:
cell->cache =
fscache_acquire_cookie(afs_cache_netfs.primary_index,
&afs_cell_cache_index_def,
cell);
Then when a volume location was accessed, it would be entered into the cell's
index and an inode would be allocated that acts as a volume type and hash chain
combination:
vlocation->cache =
fscache_acquire_cookie(cell->cache,
&afs_vlocation_cache_index_def,
vlocation);
And then a particular flavour of volume (R/O for example) could be added to
that index, creating another index for vnodes (AFS inode equivalents):
volume->cache =
fscache_acquire_cookie(vlocation->cache,
&afs_volume_cache_index_def,
volume);
======================
DATA FILE REGISTRATION
======================
The fourth step is to request a data file be created in the cache. This is
identical to index cookie acquisition. The only difference is that the type in
the object definition should be something other than index type.
vnode->cache =
fscache_acquire_cookie(volume->cache,
&afs_vnode_cache_object_def,
vnode);
=================================
MISCELLANEOUS OBJECT REGISTRATION
=================================
An optional step is to request an object of miscellaneous type be created in
the cache. This is almost identical to index cookie acquisition. The only
difference is that the type in the object definition should be something other
than index type. Whilst the parent object could be an index, it's more likely
it would be some other type of object such as a data file.
xattr->cache =
fscache_acquire_cookie(vnode->cache,
&afs_xattr_cache_object_def,
xattr);
Miscellaneous objects might be used to store extended attributes or directory
entries for example.
==========================
SETTING THE DATA FILE SIZE
==========================
The fifth step is to set the physical attributes of the file, such as its size.
This doesn't automatically reserve any space in the cache, but permits the
cache to adjust its metadata for data tracking appropriately:
int fscache_attr_changed(struct fscache_cookie *cookie);
The cache will return -ENOBUFS if there is no backing cache or if there is no
space to allocate any extra metadata required in the cache. The attributes
will be accessed with the get_attr() cookie definition operation.
Note that attempts to read or write data pages in the cache over this size may
be rebuffed with -ENOBUFS.
This operation schedules an attribute adjustment to happen asynchronously at
some point in the future, and as such, it may happen after the function returns
to the caller. The attribute adjustment excludes read and write operations.
=====================
PAGE READ/ALLOC/WRITE
=====================
And the sixth step is to store and retrieve pages in the cache. There are
three functions that are used to do this.
Note:
(1) A page should not be re-read or re-allocated without uncaching it first.
(2) A read or allocated page must be uncached when the netfs page is released
from the pagecache.
(3) A page should only be written to the cache if previous read or allocated.
This permits the cache to maintain its page tracking in proper order.
PAGE READ
---------
Firstly, the netfs should ask FS-Cache to examine the caches and read the
contents cached for a particular page of a particular file if present, or else
allocate space to store the contents if not:
typedef
void (*fscache_rw_complete_t)(struct page *page,
void *context,
int error);
int fscache_read_or_alloc_page(struct fscache_cookie *cookie,
struct page *page,
fscache_rw_complete_t end_io_func,
void *context,
gfp_t gfp);
The cookie argument must specify a cookie for an object that isn't an index,
the page specified will have the data loaded into it (and is also used to
specify the page number), and the gfp argument is used to control how any
memory allocations made are satisfied.
If the cookie indicates the inode is not cached:
(1) The function will return -ENOBUFS.
Else if there's a copy of the page resident in the cache:
(1) The mark_pages_cached() cookie operation will be called on that page.
(2) The function will submit a request to read the data from the cache's
backing device directly into the page specified.
(3) The function will return 0.
(4) When the read is complete, end_io_func() will be invoked with:
(*) The netfs data supplied when the cookie was created.
(*) The page descriptor.
(*) The context argument passed to the above function. This will be
maintained with the get_context/put_context functions mentioned above.
(*) An argument that's 0 on success or negative for an error code.
If an error occurs, it should be assumed that the page contains no usable
data.
end_io_func() will be called in process context if the read is results in
an error, but it might be called in interrupt context if the read is
successful.
Otherwise, if there's not a copy available in cache, but the cache may be able
to store the page:
(1) The mark_pages_cached() cookie operation will be called on that page.
(2) A block may be reserved in the cache and attached to the object at the
appropriate place.
(3) The function will return -ENODATA.
This function may also return -ENOMEM or -EINTR, in which case it won't have
read any data from the cache.
PAGE ALLOCATE
-------------
Alternatively, if there's not expected to be any data in the cache for a page
because the file has been extended, a block can simply be allocated instead:
int fscache_alloc_page(struct fscache_cookie *cookie,
struct page *page,
gfp_t gfp);
This is similar to the fscache_read_or_alloc_page() function, except that it
never reads from the cache. It will return 0 if a block has been allocated,
rather than -ENODATA as the other would. One or the other must be performed
before writing to the cache.
The mark_pages_cached() cookie operation will be called on the page if
successful.
PAGE WRITE
----------
Secondly, if the netfs changes the contents of the page (either due to an
initial download or if a user performs a write), then the page should be
written back to the cache:
int fscache_write_page(struct fscache_cookie *cookie,
struct page *page,
gfp_t gfp);
The cookie argument must specify a data file cookie, the page specified should
contain the data to be written (and is also used to specify the page number),
and the gfp argument is used to control how any memory allocations made are
satisfied.
The page must have first been read or allocated successfully and must not have
been uncached before writing is performed.
If the cookie indicates the inode is not cached then:
(1) The function will return -ENOBUFS.
Else if space can be allocated in the cache to hold this page:
(1) PG_fscache_write will be set on the page.
(2) The function will submit a request to write the data to cache's backing
device directly from the page specified.
(3) The function will return 0.
(4) When the write is complete PG_fscache_write is cleared on the page and
anyone waiting for that bit will be woken up.
Else if there's no space available in the cache, -ENOBUFS will be returned. It
is also possible for the PG_fscache_write bit to be cleared when no write took
place if unforeseen circumstances arose (such as a disk error).
Writing takes place asynchronously.
MULTIPLE PAGE READ
------------------
A facility is provided to read several pages at once, as requested by the
readpages() address space operation:
int fscache_read_or_alloc_pages(struct fscache_cookie *cookie,
struct address_space *mapping,
struct list_head *pages,
int *nr_pages,
fscache_rw_complete_t end_io_func,
void *context,
gfp_t gfp);
This works in a similar way to fscache_read_or_alloc_page(), except:
(1) Any page it can retrieve data for is removed from pages and nr_pages and
dispatched for reading to the disk. Reads of adjacent pages on disk may
be merged for greater efficiency.
(2) The mark_pages_cached() cookie operation will be called on several pages
at once if they're being read or allocated.
(3) If there was an general error, then that error will be returned.
Else if some pages couldn't be allocated or read, then -ENOBUFS will be
returned.
Else if some pages couldn't be read but were allocated, then -ENODATA will
be returned.
Otherwise, if all pages had reads dispatched, then 0 will be returned, the
list will be empty and *nr_pages will be 0.
(4) end_io_func will be called once for each page being read as the reads
complete. It will be called in process context if error != 0, but it may
be called in interrupt context if there is no error.
Note that a return of -ENODATA, -ENOBUFS or any other error does not preclude
some of the pages being read and some being allocated. Those pages will have
been marked appropriately and will need uncaching.
==============
PAGE UNCACHING
==============
To uncache a page, this function should be called:
void fscache_uncache_page(struct fscache_cookie *cookie,
struct page *page);
This function permits the cache to release any in-memory representation it
might be holding for this netfs page. This function must be called once for
each page on which the read or write page functions above have been called to
make sure the cache's in-memory tracking information gets torn down.
Note that pages can't be explicitly deleted from the a data file. The whole
data file must be retired (see the relinquish cookie function below).
Furthermore, note that this does not cancel the asynchronous read or write
operation started by the read/alloc and write functions, so the page
invalidation and release functions must use:
bool fscache_check_page_write(struct fscache_cookie *cookie,
struct page *page);
to see if a page is being written to the cache, and:
void fscache_wait_on_page_write(struct fscache_cookie *cookie,
struct page *page);
to wait for it to finish if it is.
==========================
INDEX AND DATA FILE UPDATE
==========================
To request an update of the index data for an index or other object, the
following function should be called:
void fscache_update_cookie(struct fscache_cookie *cookie);
This function will refer back to the netfs_data pointer stored in the cookie by
the acquisition function to obtain the data to write into each revised index
entry. The update method in the parent index definition will be called to
transfer the data.
Note that partial updates may happen automatically at other times, such as when
data blocks are added to a data file object.
===============================
MISCELLANEOUS COOKIE OPERATIONS
===============================
There are a number of operations that can be used to control cookies:
(*) Cookie pinning:
int fscache_pin_cookie(struct fscache_cookie *cookie);
void fscache_unpin_cookie(struct fscache_cookie *cookie);
These operations permit data cookies to be pinned into the cache and to
have the pinning removed. They are not permitted on index cookies.
The pinning function will return 0 if successful, -ENOBUFS in the cookie
isn't backed by a cache, -EOPNOTSUPP if the cache doesn't support pinning,
-ENOSPC if there isn't enough space to honour the operation, -ENOMEM or
-EIO if there's any other problem.
(*) Data space reservation:
int fscache_reserve_space(struct fscache_cookie *cookie, loff_t size);
This permits a netfs to request cache space be reserved to store up to the
given amount of a file. It is permitted to ask for more than the current
size of the file to allow for future file expansion.
If size is given as zero then the reservation will be cancelled.
The function will return 0 if successful, -ENOBUFS in the cookie isn't
backed by a cache, -EOPNOTSUPP if the cache doesn't support reservations,
-ENOSPC if there isn't enough space to honour the operation, -ENOMEM or
-EIO if there's any other problem.
Note that this doesn't pin an object in a cache; it can still be culled to
make space if it's not in use.
=====================
COOKIE UNREGISTRATION
=====================
To get rid of a cookie, this function should be called.
void fscache_relinquish_cookie(struct fscache_cookie *cookie,
int retire);
If retire is non-zero, then the object will be marked for recycling, and all
copies of it will be removed from all active caches in which it is present.
Not only that but all child objects will also be retired.
If retire is zero, then the object may be available again when next the
acquisition function is called. Retirement here will overrule the pinning on a
cookie.
One very important note - relinquish must NOT be called for a cookie unless all
the cookies for "child" indices, objects and pages have been relinquished
first.
================================
INDEX AND DATA FILE INVALIDATION
================================
There is no direct way to invalidate an index subtree or a data file. To do
this, the caller should relinquish and retire the cookie they have, and then
acquire a new one.
===========================
FS-CACHE SPECIFIC PAGE FLAG
===========================
FS-Cache makes use of a page flag, PG_private_2, for its own purpose. This is
given the alternative name PG_fscache.
PG_fscache is used to indicate that the page is known by the cache, and that
the cache must be informed if the page is going to go away. It's an indication
to the netfs that the cache has an interest in this page, where an interest may
be a pointer to it, resources allocated or reserved for it, or I/O in progress
upon it.
The netfs can use this information in methods such as releasepage() to
determine whether it needs to uncache a page or update it.
Furthermore, if this bit is set, releasepage() and invalidatepage() operations
will be called on a page to get rid of it, even if PG_private is not set. This
allows caching to attempted on a page before read_cache_pages() to be called
after fscache_read_or_alloc_pages() as the former will try and release pages it
was given under certain circumstances.
This bit does not overlap with such as PG_private. This means that FS-Cache
can be used with a filesystem that uses the block buffering code.
There are a number of operations defined on this flag:
int PageFsCache(struct page *page);
void SetPageFsCache(struct page *page)
void ClearPageFsCache(struct page *page)
int TestSetPageFsCache(struct page *page)
int TestClearPageFsCache(struct page *page)
These functions are bit test, bit set, bit clear, bit test and set and bit
test and clear operations on PG_fscache.

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====================================================
IN-KERNEL CACHE OBJECT REPRESENTATION AND MANAGEMENT
====================================================
By: David Howells <dhowells@redhat.com>
Contents:
(*) Representation
(*) Object management state machine.
- Provision of cpu time.
- Locking simplification.
(*) The set of states.
(*) The set of events.
==============
REPRESENTATION
==============
FS-Cache maintains an in-kernel representation of each object that a netfs is
currently interested in. Such objects are represented by the fscache_cookie
struct and are referred to as cookies.
FS-Cache also maintains a separate in-kernel representation of the objects that
a cache backend is currently actively caching. Such objects are represented by
the fscache_object struct. The cache backends allocate these upon request, and
are expected to embed them in their own representations. These are referred to
as objects.
There is a 1:N relationship between cookies and objects. A cookie may be
represented by multiple objects - an index may exist in more than one cache -
or even by no objects (it may not be cached).
Furthermore, both cookies and objects are hierarchical. The two hierarchies
correspond, but the cookies tree is a superset of the union of the object trees
of multiple caches:
NETFS INDEX TREE : CACHE 1 : CACHE 2
: :
: +-----------+ :
+----------->| IObject | :
+-----------+ | : +-----------+ :
| ICookie |-------+ : | :
+-----------+ | : | : +-----------+
| +------------------------------>| IObject |
| : | : +-----------+
| : V : |
| : +-----------+ : |
V +----------->| IObject | : |
+-----------+ | : +-----------+ : |
| ICookie |-------+ : | : V
+-----------+ | : | : +-----------+
| +------------------------------>| IObject |
+-----+-----+ : | : +-----------+
| | : | : |
V | : V : |
+-----------+ | : +-----------+ : |
| ICookie |------------------------->| IObject | : |
+-----------+ | : +-----------+ : |
| V : | : V
| +-----------+ : | : +-----------+
| | ICookie |-------------------------------->| IObject |
| +-----------+ : | : +-----------+
V | : V : |
+-----------+ | : +-----------+ : |
| DCookie |------------------------->| DObject | : |
+-----------+ | : +-----------+ : |
| : : |
+-------+-------+ : : |
| | : : |
V V : : V
+-----------+ +-----------+ : : +-----------+
| DCookie | | DCookie |------------------------>| DObject |
+-----------+ +-----------+ : : +-----------+
: :
In the above illustration, ICookie and IObject represent indices and DCookie
and DObject represent data storage objects. Indices may have representation in
multiple caches, but currently, non-index objects may not. Objects of any type
may also be entirely unrepresented.
As far as the netfs API goes, the netfs is only actually permitted to see
pointers to the cookies. The cookies themselves and any objects attached to
those cookies are hidden from it.
===============================
OBJECT MANAGEMENT STATE MACHINE
===============================
Within FS-Cache, each active object is managed by its own individual state
machine. The state for an object is kept in the fscache_object struct, in
object->state. A cookie may point to a set of objects that are in different
states.
Each state has an action associated with it that is invoked when the machine
wakes up in that state. There are four logical sets of states:
(1) Preparation: states that wait for the parent objects to become ready. The
representations are hierarchical, and it is expected that an object must
be created or accessed with respect to its parent object.
(2) Initialisation: states that perform lookups in the cache and validate
what's found and that create on disk any missing metadata.
(3) Normal running: states that allow netfs operations on objects to proceed
and that update the state of objects.
(4) Termination: states that detach objects from their netfs cookies, that
delete objects from disk, that handle disk and system errors and that free
up in-memory resources.
In most cases, transitioning between states is in response to signalled events.
When a state has finished processing, it will usually set the mask of events in
which it is interested (object->event_mask) and relinquish the worker thread.
Then when an event is raised (by calling fscache_raise_event()), if the event
is not masked, the object will be queued for processing (by calling
fscache_enqueue_object()).
PROVISION OF CPU TIME
---------------------
The work to be done by the various states is given CPU time by the threads of
the slow work facility (see Documentation/slow-work.txt). This is used in
preference to the workqueue facility because:
(1) Threads may be completely occupied for very long periods of time by a
particular work item. These state actions may be doing sequences of
synchronous, journalled disk accesses (lookup, mkdir, create, setxattr,
getxattr, truncate, unlink, rmdir, rename).
(2) Threads may do little actual work, but may rather spend a lot of time
sleeping on I/O. This means that single-threaded and 1-per-CPU-threaded
workqueues don't necessarily have the right numbers of threads.
LOCKING SIMPLIFICATION
----------------------
Because only one worker thread may be operating on any particular object's
state machine at once, this simplifies the locking, particularly with respect
to disconnecting the netfs's representation of a cache object (fscache_cookie)
from the cache backend's representation (fscache_object) - which may be
requested from either end.
=================
THE SET OF STATES
=================
The object state machine has a set of states that it can be in. There are
preparation states in which the object sets itself up and waits for its parent
object to transit to a state that allows access to its children:
(1) State FSCACHE_OBJECT_INIT.
Initialise the object and wait for the parent object to become active. In
the cache, it is expected that it will not be possible to look an object
up from the parent object, until that parent object itself has been looked
up.
There are initialisation states in which the object sets itself up and accesses
disk for the object metadata:
(2) State FSCACHE_OBJECT_LOOKING_UP.
Look up the object on disk, using the parent as a starting point.
FS-Cache expects the cache backend to probe the cache to see whether this
object is represented there, and if it is, to see if it's valid (coherency
management).
The cache should call fscache_object_lookup_negative() to indicate lookup
failure for whatever reason, and should call fscache_obtained_object() to
indicate success.
At the completion of lookup, FS-Cache will let the netfs go ahead with
read operations, no matter whether the file is yet cached. If not yet
cached, read operations will be immediately rejected with ENODATA until
the first known page is uncached - as to that point there can be no data
to be read out of the cache for that file that isn't currently also held
in the pagecache.
(3) State FSCACHE_OBJECT_CREATING.
Create an object on disk, using the parent as a starting point. This
happens if the lookup failed to find the object, or if the object's
coherency data indicated what's on disk is out of date. In this state,
FS-Cache expects the cache to create
The cache should call fscache_obtained_object() if creation completes
successfully, fscache_object_lookup_negative() otherwise.
At the completion of creation, FS-Cache will start processing write
operations the netfs has queued for an object. If creation failed, the
write ops will be transparently discarded, and nothing recorded in the
cache.
There are some normal running states in which the object spends its time
servicing netfs requests:
(4) State FSCACHE_OBJECT_AVAILABLE.
A transient state in which pending operations are started, child objects
are permitted to advance from FSCACHE_OBJECT_INIT state, and temporary
lookup data is freed.
(5) State FSCACHE_OBJECT_ACTIVE.
The normal running state. In this state, requests the netfs makes will be
passed on to the cache.
(6) State FSCACHE_OBJECT_UPDATING.
The state machine comes here to update the object in the cache from the
netfs's records. This involves updating the auxiliary data that is used
to maintain coherency.
And there are terminal states in which an object cleans itself up, deallocates
memory and potentially deletes stuff from disk:
(7) State FSCACHE_OBJECT_LC_DYING.
The object comes here if it is dying because of a lookup or creation
error. This would be due to a disk error or system error of some sort.
Temporary data is cleaned up, and the parent is released.
(8) State FSCACHE_OBJECT_DYING.
The object comes here if it is dying due to an error, because its parent
cookie has been relinquished by the netfs or because the cache is being
withdrawn.
Any child objects waiting on this one are given CPU time so that they too
can destroy themselves. This object waits for all its children to go away
before advancing to the next state.
(9) State FSCACHE_OBJECT_ABORT_INIT.
The object comes to this state if it was waiting on its parent in
FSCACHE_OBJECT_INIT, but its parent died. The object will destroy itself
so that the parent may proceed from the FSCACHE_OBJECT_DYING state.
(10) State FSCACHE_OBJECT_RELEASING.
(11) State FSCACHE_OBJECT_RECYCLING.
The object comes to one of these two states when dying once it is rid of
all its children, if it is dying because the netfs relinquished its
cookie. In the first state, the cached data is expected to persist, and
in the second it will be deleted.
(12) State FSCACHE_OBJECT_WITHDRAWING.
The object transits to this state if the cache decides it wants to
withdraw the object from service, perhaps to make space, but also due to
error or just because the whole cache is being withdrawn.
(13) State FSCACHE_OBJECT_DEAD.
The object transits to this state when the in-memory object record is
ready to be deleted. The object processor shouldn't ever see an object in
this state.
THE SET OF EVENTS
-----------------
There are a number of events that can be raised to an object state machine:
(*) FSCACHE_OBJECT_EV_UPDATE
The netfs requested that an object be updated. The state machine will ask
the cache backend to update the object, and the cache backend will ask the
netfs for details of the change through its cookie definition ops.
(*) FSCACHE_OBJECT_EV_CLEARED
This is signalled in two circumstances:
(a) when an object's last child object is dropped and
(b) when the last operation outstanding on an object is completed.
This is used to proceed from the dying state.
(*) FSCACHE_OBJECT_EV_ERROR
This is signalled when an I/O error occurs during the processing of some
object.
(*) FSCACHE_OBJECT_EV_RELEASE
(*) FSCACHE_OBJECT_EV_RETIRE
These are signalled when the netfs relinquishes a cookie it was using.
The event selected depends on whether the netfs asks for the backing
object to be retired (deleted) or retained.
(*) FSCACHE_OBJECT_EV_WITHDRAW
This is signalled when the cache backend wants to withdraw an object.
This means that the object will have to be detached from the netfs's
cookie.
Because the withdrawing releasing/retiring events are all handled by the object
state machine, it doesn't matter if there's a collision with both ends trying
to sever the connection at the same time. The state machine can just pick
which one it wants to honour, and that effects the other.

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================================
ASYNCHRONOUS OPERATIONS HANDLING
================================
By: David Howells <dhowells@redhat.com>
Contents:
(*) Overview.
(*) Operation record initialisation.
(*) Parameters.
(*) Procedure.
(*) Asynchronous callback.
========
OVERVIEW
========
FS-Cache has an asynchronous operations handling facility that it uses for its
data storage and retrieval routines. Its operations are represented by
fscache_operation structs, though these are usually embedded into some other
structure.
This facility is available to and expected to be be used by the cache backends,
and FS-Cache will create operations and pass them off to the appropriate cache
backend for completion.
To make use of this facility, <linux/fscache-cache.h> should be #included.
===============================
OPERATION RECORD INITIALISATION
===============================
An operation is recorded in an fscache_operation struct:
struct fscache_operation {
union {
struct work_struct fast_work;
struct slow_work slow_work;
};
unsigned long flags;
fscache_operation_processor_t processor;
...
};
Someone wanting to issue an operation should allocate something with this
struct embedded in it. They should initialise it by calling:
void fscache_operation_init(struct fscache_operation *op,
fscache_operation_release_t release);
with the operation to be initialised and the release function to use.
The op->flags parameter should be set to indicate the CPU time provision and
the exclusivity (see the Parameters section).
The op->fast_work, op->slow_work and op->processor flags should be set as
appropriate for the CPU time provision (see the Parameters section).
FSCACHE_OP_WAITING may be set in op->flags prior to each submission of the
operation and waited for afterwards.
==========
PARAMETERS
==========
There are a number of parameters that can be set in the operation record's flag
parameter. There are three options for the provision of CPU time in these
operations:
(1) The operation may be done synchronously (FSCACHE_OP_MYTHREAD). A thread
may decide it wants to handle an operation itself without deferring it to
another thread.
This is, for example, used in read operations for calling readpages() on
the backing filesystem in CacheFiles. Although readpages() does an
asynchronous data fetch, the determination of whether pages exist is done
synchronously - and the netfs does not proceed until this has been
determined.
If this option is to be used, FSCACHE_OP_WAITING must be set in op->flags
before submitting the operation, and the operating thread must wait for it
to be cleared before proceeding:
wait_on_bit(&op->flags, FSCACHE_OP_WAITING,
fscache_wait_bit, TASK_UNINTERRUPTIBLE);
(2) The operation may be fast asynchronous (FSCACHE_OP_FAST), in which case it
will be given to keventd to process. Such an operation is not permitted
to sleep on I/O.
This is, for example, used by CacheFiles to copy data from a backing fs
page to a netfs page after the backing fs has read the page in.
If this option is used, op->fast_work and op->processor must be
initialised before submitting the operation:
INIT_WORK(&op->fast_work, do_some_work);
(3) The operation may be slow asynchronous (FSCACHE_OP_SLOW), in which case it
will be given to the slow work facility to process. Such an operation is
permitted to sleep on I/O.
This is, for example, used by FS-Cache to handle background writes of
pages that have just been fetched from a remote server.
If this option is used, op->slow_work and op->processor must be
initialised before submitting the operation:
fscache_operation_init_slow(op, processor)
Furthermore, operations may be one of two types:
(1) Exclusive (FSCACHE_OP_EXCLUSIVE). Operations of this type may not run in
conjunction with any other operation on the object being operated upon.
An example of this is the attribute change operation, in which the file
being written to may need truncation.
(2) Shareable. Operations of this type may be running simultaneously. It's
up to the operation implementation to prevent interference between other
operations running at the same time.
=========
PROCEDURE
=========
Operations are used through the following procedure:
(1) The submitting thread must allocate the operation and initialise it
itself. Normally this would be part of a more specific structure with the
generic op embedded within.
(2) The submitting thread must then submit the operation for processing using
one of the following two functions:
int fscache_submit_op(struct fscache_object *object,
struct fscache_operation *op);
int fscache_submit_exclusive_op(struct fscache_object *object,
struct fscache_operation *op);
The first function should be used to submit non-exclusive ops and the
second to submit exclusive ones. The caller must still set the
FSCACHE_OP_EXCLUSIVE flag.
If successful, both functions will assign the operation to the specified
object and return 0. -ENOBUFS will be returned if the object specified is
permanently unavailable.
The operation manager will defer operations on an object that is still
undergoing lookup or creation. The operation will also be deferred if an
operation of conflicting exclusivity is in progress on the object.
If the operation is asynchronous, the manager will retain a reference to
it, so the caller should put their reference to it by passing it to:
void fscache_put_operation(struct fscache_operation *op);
(3) If the submitting thread wants to do the work itself, and has marked the
operation with FSCACHE_OP_MYTHREAD, then it should monitor
FSCACHE_OP_WAITING as described above and check the state of the object if
necessary (the object might have died whilst the thread was waiting).
When it has finished doing its processing, it should call
fscache_put_operation() on it.
(4) The operation holds an effective lock upon the object, preventing other
exclusive ops conflicting until it is released. The operation can be
enqueued for further immediate asynchronous processing by adjusting the
CPU time provisioning option if necessary, eg:
op->flags &= ~FSCACHE_OP_TYPE;
op->flags |= ~FSCACHE_OP_FAST;
and calling:
void fscache_enqueue_operation(struct fscache_operation *op)
This can be used to allow other things to have use of the worker thread
pools.
=====================
ASYNCHRONOUS CALLBACK
=====================
When used in asynchronous mode, the worker thread pool will invoke the
processor method with a pointer to the operation. This should then get at the
container struct by using container_of():
static void fscache_write_op(struct fscache_operation *_op)
{
struct fscache_storage *op =
container_of(_op, struct fscache_storage, op);
...
}
The caller holds a reference on the operation, and will invoke
fscache_put_operation() when the processor function returns. The processor
function is at liberty to call fscache_enqueue_operation() or to take extra
references.