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[PATCH] update some mm/ comments

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Let's try to keep mm/ comments more useful and up to date. This is a start.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Nick Piggin
2006-09-25 23:31:35 -07:00
committed by Linus Torvalds
parent e5ac9c5aec
commit da6052f7b3
3 changed files with 64 additions and 47 deletions
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68
include/linux/mm.h
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@ -219,7 +219,8 @@ struct inode;
* Each physical page in the system has a struct page associated with
* it to keep track of whatever it is we are using the page for at the
* moment. Note that we have no way to track which tasks are using
* a page.
* a page, though if it is a pagecache page, rmap structures can tell us
* who is mapping it.
*/
struct page {
unsigned long flags; /* Atomic flags, some possibly
@ -299,8 +300,7 @@ struct page {
*/
/*
* Drop a ref, return true if the logical refcount fell to zero (the page has
* no users)
* Drop a ref, return true if the refcount fell to zero (the page has no users)
*/
static inline int put_page_testzero(struct page *page)
{
@ -356,43 +356,55 @@ void split_page(struct page *page, unsigned int order);
* For the non-reserved pages, page_count(page) denotes a reference count.
* page_count() == 0 means the page is free. page->lru is then used for
* freelist management in the buddy allocator.
* page_count() == 1 means the page is used for exactly one purpose
* (e.g. a private data page of one process).
* page_count() > 0 means the page has been allocated.
*
* A page may be used for kmalloc() or anyone else who does a
* __get_free_page(). In this case the page_count() is at least 1, and
* all other fields are unused but should be 0 or NULL. The
* management of this page is the responsibility of the one who uses
* it.
* Pages are allocated by the slab allocator in order to provide memory
* to kmalloc and kmem_cache_alloc. In this case, the management of the
* page, and the fields in 'struct page' are the responsibility of mm/slab.c
* unless a particular usage is carefully commented. (the responsibility of
* freeing the kmalloc memory is the caller's, of course).
*
* The other pages (we may call them "process pages") are completely
* A page may be used by anyone else who does a __get_free_page().
* In this case, page_count still tracks the references, and should only
* be used through the normal accessor functions. The top bits of page->flags
* and page->virtual store page management information, but all other fields
* are unused and could be used privately, carefully. The management of this
* page is the responsibility of the one who allocated it, and those who have
* subsequently been given references to it.
*
* The other pages (we may call them "pagecache pages") are completely
* managed by the Linux memory manager: I/O, buffers, swapping etc.
* The following discussion applies only to them.
*
* A page may belong to an inode's memory mapping. In this case,
* page->mapping is the pointer to the inode, and page->index is the
* file offset of the page, in units of PAGE_CACHE_SIZE.
* A pagecache page contains an opaque `private' member, which belongs to the
* page's address_space. Usually, this is the address of a circular list of
* the page's disk buffers. PG_private must be set to tell the VM to call
* into the filesystem to release these pages.
*
* A page contains an opaque `private' member, which belongs to the
* page's address_space. Usually, this is the address of a circular
* list of the page's disk buffers.
* A page may belong to an inode's memory mapping. In this case, page->mapping
* is the pointer to the inode, and page->index is the file offset of the page,
* in units of PAGE_CACHE_SIZE.
*
* For pages belonging to inodes, the page_count() is the number of
* attaches, plus 1 if `private' contains something, plus one for
* the page cache itself.
* If pagecache pages are not associated with an inode, they are said to be
* anonymous pages. These may become associated with the swapcache, and in that
* case PG_swapcache is set, and page->private is an offset into the swapcache.
*
* Instead of keeping dirty/clean pages in per address-space lists, we instead
* now tag pages as dirty/under writeback in the radix tree.
* In either case (swapcache or inode backed), the pagecache itself holds one
* reference to the page. Setting PG_private should also increment the
* refcount. The each user mapping also has a reference to the page.
*
* There is also a per-mapping radix tree mapping index to the page
* in memory if present. The tree is rooted at mapping->root.
* The pagecache pages are stored in a per-mapping radix tree, which is
* rooted at mapping->page_tree, and indexed by offset.
* Where 2.4 and early 2.6 kernels kept dirty/clean pages in per-address_space
* lists, we instead now tag pages as dirty/writeback in the radix tree.
*
* All process pages can do I/O:
* All pagecache pages may be subject to I/O:
* - inode pages may need to be read from disk,
* - inode pages which have been modified and are MAP_SHARED may need
* to be written to disk,
* - private pages which have been modified may need to be swapped out
* to swap space and (later) to be read back into memory.
* to be written back to the inode on disk,
* - anonymous pages (including MAP_PRIVATE file mappings) which have been
* modified may need to be swapped out to swap space and (later) to be read
* back into memory.
*/
/*