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NFS: Fix a race when doing NFS write coalescing

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Currently we do write coalescing in a very inefficient manner: one pass in
generic_writepages() in order to lock the pages for writing, then one pass
in nfs_flush_mapping() and/or nfs_sync_mapping_wait() in order to gather
the locked pages for coalescing into RPC requests of size "wsize".

In fact, it turns out there is actually a deadlock possible here since we
only start I/O on the second pass. If the user signals the process while
we're in nfs_sync_mapping_wait(), for instance, then we may exit before
starting I/O on all the requests that have been queued up.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
This commit is contained in:
Trond Myklebust
2007-04-02 19:29:52 -04:00
parent 8b09bee308
commit c63c7b0513
4 changed files with 50 additions and 201 deletions
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92
fs/nfs/pagelist.c
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@@ -17,7 +17,6 @@
#include <linux/nfs_page.h>
#include <linux/nfs_fs.h>
#include <linux/nfs_mount.h>
#include <linux/writeback.h>
#define NFS_PARANOIA 1
@@ -353,25 +352,6 @@ int nfs_pageio_add_request(struct nfs_pageio_descriptor *desc,
return 1;
}
/**
* nfs_pageio_add_list - Split coalesced requests out from a list.
* @desc: destination io descriptor
* @head: source list
*
* Moves a maximum of 'nmax' elements from one list to another.
* The elements are checked to ensure that they form a contiguous set
* of pages, and that the RPC credentials are the same.
*/
void nfs_pageio_add_list(struct nfs_pageio_descriptor *desc,
struct list_head *head)
{
while (!list_empty(head)) {
struct nfs_page *req = nfs_list_entry(head->next);
if (!nfs_pageio_add_request(desc, req))
break;
}
}
/**
* nfs_pageio_complete - Complete I/O on an nfs_pageio_descriptor
* @desc: pointer to io descriptor
@@ -382,78 +362,6 @@ void nfs_pageio_complete(struct nfs_pageio_descriptor *desc)
}
#define NFS_SCAN_MAXENTRIES 16
/**
* nfs_scan_dirty - Scan the radix tree for dirty requests
* @mapping: pointer to address space
* @wbc: writeback_control structure
* @dst: Destination list
*
* Moves elements from one of the inode request lists.
* If the number of requests is set to 0, the entire address_space
* starting at index idx_start, is scanned.
* The requests are *not* checked to ensure that they form a contiguous set.
* You must be holding the inode's req_lock when calling this function
*/
long nfs_scan_dirty(struct address_space *mapping,
struct writeback_control *wbc,
struct list_head *dst)
{
struct nfs_inode *nfsi = NFS_I(mapping->host);
struct nfs_page *pgvec[NFS_SCAN_MAXENTRIES];
struct nfs_page *req;
pgoff_t idx_start, idx_end;
long res = 0;
int found, i;
if (nfsi->ndirty == 0)
return 0;
if (wbc->range_cyclic) {
idx_start = 0;
idx_end = ULONG_MAX;
} else if (wbc->range_end == 0) {
idx_start = wbc->range_start >> PAGE_CACHE_SHIFT;
idx_end = ULONG_MAX;
} else {
idx_start = wbc->range_start >> PAGE_CACHE_SHIFT;
idx_end = wbc->range_end >> PAGE_CACHE_SHIFT;
}
for (;;) {
unsigned int toscan = NFS_SCAN_MAXENTRIES;
found = radix_tree_gang_lookup_tag(&nfsi->nfs_page_tree,
(void **)&pgvec[0], idx_start, toscan,
NFS_PAGE_TAG_DIRTY);
/* Did we make progress? */
if (found <= 0)
break;
for (i = 0; i < found; i++) {
req = pgvec[i];
if (!wbc->range_cyclic && req->wb_index > idx_end)
goto out;
/* Try to lock request and mark it for writeback */
if (!nfs_set_page_writeback_locked(req))
goto next;
radix_tree_tag_clear(&nfsi->nfs_page_tree,
req->wb_index, NFS_PAGE_TAG_DIRTY);
nfsi->ndirty--;
nfs_list_remove_request(req);
nfs_list_add_request(req, dst);
res++;
if (res == LONG_MAX)
goto out;
next:
idx_start = req->wb_index + 1;
}
}
out:
WARN_ON ((nfsi->ndirty == 0) != list_empty(&nfsi->dirty));
return res;
}
/**
* nfs_scan_list - Scan a list for matching requests
* @nfsi: NFS inode