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authorDavid Howells <dhowells@redhat.com>2009-11-19 18:11:35 +0000
committerDavid Howells <dhowells@redhat.com>2009-11-19 18:11:35 +0000
commit201a15428bd54f83eccec8b7c64a04b8f9431204 (patch)
tree326fcce64ce96657253fd141a3f4a767ac95418a /fs
parente3d4d28b1c8cc7c26536a50b43d86ccd39878550 (diff)
FS-Cache: Handle pages pending storage that get evicted under OOM conditions
Handle netfs pages that the vmscan algorithm wants to evict from the pagecache under OOM conditions, but that are waiting for write to the cache. Under these conditions, vmscan calls the releasepage() function of the netfs, asking if a page can be discarded. The problem is typified by the following trace of a stuck process: kslowd005 D 0000000000000000 0 4253 2 0x00000080 ffff88001b14f370 0000000000000046 ffff880020d0d000 0000000000000007 0000000000000006 0000000000000001 ffff88001b14ffd8 ffff880020d0d2a8 000000000000ddf0 00000000000118c0 00000000000118c0 ffff880020d0d2a8 Call Trace: [<ffffffffa00782d8>] __fscache_wait_on_page_write+0x8b/0xa7 [fscache] [<ffffffff8104c0f1>] ? autoremove_wake_function+0x0/0x34 [<ffffffffa0078240>] ? __fscache_check_page_write+0x63/0x70 [fscache] [<ffffffffa00b671d>] nfs_fscache_release_page+0x4e/0xc4 [nfs] [<ffffffffa00927f0>] nfs_release_page+0x3c/0x41 [nfs] [<ffffffff810885d3>] try_to_release_page+0x32/0x3b [<ffffffff81093203>] shrink_page_list+0x316/0x4ac [<ffffffff8109372b>] shrink_inactive_list+0x392/0x67c [<ffffffff813532fa>] ? __mutex_unlock_slowpath+0x100/0x10b [<ffffffff81058df0>] ? trace_hardirqs_on_caller+0x10c/0x130 [<ffffffff8135330e>] ? mutex_unlock+0x9/0xb [<ffffffff81093aa2>] shrink_list+0x8d/0x8f [<ffffffff81093d1c>] shrink_zone+0x278/0x33c [<ffffffff81052d6c>] ? ktime_get_ts+0xad/0xba [<ffffffff81094b13>] try_to_free_pages+0x22e/0x392 [<ffffffff81091e24>] ? isolate_pages_global+0x0/0x212 [<ffffffff8108e743>] __alloc_pages_nodemask+0x3dc/0x5cf [<ffffffff81089529>] grab_cache_page_write_begin+0x65/0xaa [<ffffffff8110f8c0>] ext3_write_begin+0x78/0x1eb [<ffffffff81089ec5>] generic_file_buffered_write+0x109/0x28c [<ffffffff8103cb69>] ? current_fs_time+0x22/0x29 [<ffffffff8108a509>] __generic_file_aio_write+0x350/0x385 [<ffffffff8108a588>] ? generic_file_aio_write+0x4a/0xae [<ffffffff8108a59e>] generic_file_aio_write+0x60/0xae [<ffffffff810b2e82>] do_sync_write+0xe3/0x120 [<ffffffff8104c0f1>] ? autoremove_wake_function+0x0/0x34 [<ffffffff810b18e1>] ? __dentry_open+0x1a5/0x2b8 [<ffffffff810b1a76>] ? dentry_open+0x82/0x89 [<ffffffffa00e693c>] cachefiles_write_page+0x298/0x335 [cachefiles] [<ffffffffa0077147>] fscache_write_op+0x178/0x2c2 [fscache] [<ffffffffa0075656>] fscache_op_execute+0x7a/0xd1 [fscache] [<ffffffff81082093>] slow_work_execute+0x18f/0x2d1 [<ffffffff8108239a>] slow_work_thread+0x1c5/0x308 [<ffffffff8104c0f1>] ? autoremove_wake_function+0x0/0x34 [<ffffffff810821d5>] ? slow_work_thread+0x0/0x308 [<ffffffff8104be91>] kthread+0x7a/0x82 [<ffffffff8100beda>] child_rip+0xa/0x20 [<ffffffff8100b87c>] ? restore_args+0x0/0x30 [<ffffffff8102ef83>] ? tg_shares_up+0x171/0x227 [<ffffffff8104be17>] ? kthread+0x0/0x82 [<ffffffff8100bed0>] ? child_rip+0x0/0x20 In the above backtrace, the following is happening: (1) A page storage operation is being executed by a slow-work thread (fscache_write_op()). (2) FS-Cache farms the operation out to the cache to perform (cachefiles_write_page()). (3) CacheFiles is then calling Ext3 to perform the actual write, using Ext3's standard write (do_sync_write()) under KERNEL_DS directly from the netfs page. (4) However, for Ext3 to perform the write, it must allocate some memory, in particular, it must allocate at least one page cache page into which it can copy the data from the netfs page. (5) Under OOM conditions, the memory allocator can't immediately come up with a page, so it uses vmscan to find something to discard (try_to_free_pages()). (6) vmscan finds a clean netfs page it might be able to discard (possibly the one it's trying to write out). (7) The netfs is called to throw the page away (nfs_release_page()) - but it's called with __GFP_WAIT, so the netfs decides to wait for the store to complete (__fscache_wait_on_page_write()). (8) This blocks a slow-work processing thread - possibly against itself. The system ends up stuck because it can't write out any netfs pages to the cache without allocating more memory. To avoid this, we make FS-Cache cancel some writes that aren't in the middle of actually being performed. This means that some data won't make it into the cache this time. To support this, a new FS-Cache function is added fscache_maybe_release_page() that replaces what the netfs releasepage() functions used to do with respect to the cache. The decisions fscache_maybe_release_page() makes are counted and displayed through /proc/fs/fscache/stats on a line labelled "VmScan". There are four counters provided: "nos=N" - pages that weren't pending storage; "gon=N" - pages that were pending storage when we first looked, but weren't by the time we got the object lock; "bsy=N" - pages that we ignored as they were actively being written when we looked; and "can=N" - pages that we cancelled the storage of. What I'd really like to do is alter the behaviour of the cancellation heuristics, depending on how necessary it is to expel pages. If there are plenty of other pages that aren't waiting to be written to the cache that could be ejected first, then it would be nice to hold up on immediate cancellation of cache writes - but I don't see a way of doing that. Signed-off-by: David Howells <dhowells@redhat.com>
Diffstat (limited to 'fs')
-rw-r--r--fs/9p/cache.c14
-rw-r--r--fs/afs/file.c15
-rw-r--r--fs/fscache/internal.h5
-rw-r--r--fs/fscache/page.c79
-rw-r--r--fs/fscache/stats.c11
-rw-r--r--fs/nfs/fscache.c10
6 files changed, 100 insertions, 34 deletions
diff --git a/fs/9p/cache.c b/fs/9p/cache.c
index 51c94e26a34..bcc5357a906 100644
--- a/fs/9p/cache.c
+++ b/fs/9p/cache.c
@@ -343,18 +343,7 @@ int __v9fs_fscache_release_page(struct page *page, gfp_t gfp)
BUG_ON(!vcookie->fscache);
- if (PageFsCache(page)) {
- if (fscache_check_page_write(vcookie->fscache, page)) {
- if (!(gfp & __GFP_WAIT))
- return 0;
- fscache_wait_on_page_write(vcookie->fscache, page);
- }
-
- fscache_uncache_page(vcookie->fscache, page);
- ClearPageFsCache(page);
- }
-
- return 1;
+ return fscache_maybe_release_page(vnode->cache, page, gfp);
}
void __v9fs_fscache_invalidate_page(struct page *page)
@@ -368,7 +357,6 @@ void __v9fs_fscache_invalidate_page(struct page *page)
fscache_wait_on_page_write(vcookie->fscache, page);
BUG_ON(!PageLocked(page));
fscache_uncache_page(vcookie->fscache, page);
- ClearPageFsCache(page);
}
}
diff --git a/fs/afs/file.c b/fs/afs/file.c
index 681c2a7b013..39b301662f2 100644
--- a/fs/afs/file.c
+++ b/fs/afs/file.c
@@ -315,7 +315,6 @@ static void afs_invalidatepage(struct page *page, unsigned long offset)
struct afs_vnode *vnode = AFS_FS_I(page->mapping->host);
fscache_wait_on_page_write(vnode->cache, page);
fscache_uncache_page(vnode->cache, page);
- ClearPageFsCache(page);
}
#endif
@@ -349,17 +348,9 @@ static int afs_releasepage(struct page *page, gfp_t gfp_flags)
/* deny if page is being written to the cache and the caller hasn't
* elected to wait */
#ifdef CONFIG_AFS_FSCACHE
- if (PageFsCache(page)) {
- if (fscache_check_page_write(vnode->cache, page)) {
- if (!(gfp_flags & __GFP_WAIT)) {
- _leave(" = F [cache busy]");
- return 0;
- }
- fscache_wait_on_page_write(vnode->cache, page);
- }
-
- fscache_uncache_page(vnode->cache, page);
- ClearPageFsCache(page);
+ if (!fscache_maybe_release_page(vnode->cache, page, gfp_flags)) {
+ _leave(" = F [cache busy]");
+ return 0;
}
#endif
diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h
index a0769872b19..e5046519b15 100644
--- a/fs/fscache/internal.h
+++ b/fs/fscache/internal.h
@@ -180,6 +180,11 @@ extern atomic_t fscache_n_store_pages;
extern atomic_t fscache_n_store_radix_deletes;
extern atomic_t fscache_n_store_pages_over_limit;
+extern atomic_t fscache_n_store_vmscan_not_storing;
+extern atomic_t fscache_n_store_vmscan_gone;
+extern atomic_t fscache_n_store_vmscan_busy;
+extern atomic_t fscache_n_store_vmscan_cancelled;
+
extern atomic_t fscache_n_marks;
extern atomic_t fscache_n_uncaches;
diff --git a/fs/fscache/page.c b/fs/fscache/page.c
index 022a5da8e13..fc76798bd96 100644
--- a/fs/fscache/page.c
+++ b/fs/fscache/page.c
@@ -43,6 +43,75 @@ void __fscache_wait_on_page_write(struct fscache_cookie *cookie, struct page *pa
EXPORT_SYMBOL(__fscache_wait_on_page_write);
/*
+ * decide whether a page can be released, possibly by cancelling a store to it
+ * - we're allowed to sleep if __GFP_WAIT is flagged
+ */
+bool __fscache_maybe_release_page(struct fscache_cookie *cookie,
+ struct page *page,
+ gfp_t gfp)
+{
+ struct page *xpage;
+ void *val;
+
+ _enter("%p,%p,%x", cookie, page, gfp);
+
+ rcu_read_lock();
+ val = radix_tree_lookup(&cookie->stores, page->index);
+ if (!val) {
+ rcu_read_unlock();
+ fscache_stat(&fscache_n_store_vmscan_not_storing);
+ __fscache_uncache_page(cookie, page);
+ return true;
+ }
+
+ /* see if the page is actually undergoing storage - if so we can't get
+ * rid of it till the cache has finished with it */
+ if (radix_tree_tag_get(&cookie->stores, page->index,
+ FSCACHE_COOKIE_STORING_TAG)) {
+ rcu_read_unlock();
+ goto page_busy;
+ }
+
+ /* the page is pending storage, so we attempt to cancel the store and
+ * discard the store request so that the page can be reclaimed */
+ spin_lock(&cookie->stores_lock);
+ rcu_read_unlock();
+
+ if (radix_tree_tag_get(&cookie->stores, page->index,
+ FSCACHE_COOKIE_STORING_TAG)) {
+ /* the page started to undergo storage whilst we were looking,
+ * so now we can only wait or return */
+ spin_unlock(&cookie->stores_lock);
+ goto page_busy;
+ }
+
+ xpage = radix_tree_delete(&cookie->stores, page->index);
+ spin_unlock(&cookie->stores_lock);
+
+ if (xpage) {
+ fscache_stat(&fscache_n_store_vmscan_cancelled);
+ fscache_stat(&fscache_n_store_radix_deletes);
+ ASSERTCMP(xpage, ==, page);
+ } else {
+ fscache_stat(&fscache_n_store_vmscan_gone);
+ }
+
+ wake_up_bit(&cookie->flags, 0);
+ if (xpage)
+ page_cache_release(xpage);
+ __fscache_uncache_page(cookie, page);
+ return true;
+
+page_busy:
+ /* we might want to wait here, but that could deadlock the allocator as
+ * the slow-work threads writing to the cache may all end up sleeping
+ * on memory allocation */
+ fscache_stat(&fscache_n_store_vmscan_busy);
+ return false;
+}
+EXPORT_SYMBOL(__fscache_maybe_release_page);
+
+/*
* note that a page has finished being written to the cache
*/
static void fscache_end_page_write(struct fscache_object *object,
@@ -57,6 +126,8 @@ static void fscache_end_page_write(struct fscache_object *object,
/* delete the page from the tree if it is now no longer
* pending */
spin_lock(&cookie->stores_lock);
+ radix_tree_tag_clear(&cookie->stores, page->index,
+ FSCACHE_COOKIE_STORING_TAG);
if (!radix_tree_tag_get(&cookie->stores, page->index,
FSCACHE_COOKIE_PENDING_TAG)) {
fscache_stat(&fscache_n_store_radix_deletes);
@@ -640,8 +711,12 @@ static void fscache_write_op(struct fscache_operation *_op)
goto superseded;
}
- radix_tree_tag_clear(&cookie->stores, page->index,
- FSCACHE_COOKIE_PENDING_TAG);
+ if (page) {
+ radix_tree_tag_set(&cookie->stores, page->index,
+ FSCACHE_COOKIE_STORING_TAG);
+ radix_tree_tag_clear(&cookie->stores, page->index,
+ FSCACHE_COOKIE_PENDING_TAG);
+ }
spin_unlock(&cookie->stores_lock);
spin_unlock(&object->lock);
diff --git a/fs/fscache/stats.c b/fs/fscache/stats.c
index 045ba396dbf..cda69994e06 100644
--- a/fs/fscache/stats.c
+++ b/fs/fscache/stats.c
@@ -63,6 +63,11 @@ atomic_t fscache_n_store_pages;
atomic_t fscache_n_store_radix_deletes;
atomic_t fscache_n_store_pages_over_limit;
+atomic_t fscache_n_store_vmscan_not_storing;
+atomic_t fscache_n_store_vmscan_gone;
+atomic_t fscache_n_store_vmscan_busy;
+atomic_t fscache_n_store_vmscan_cancelled;
+
atomic_t fscache_n_marks;
atomic_t fscache_n_uncaches;
@@ -211,6 +216,12 @@ static int fscache_stats_show(struct seq_file *m, void *v)
atomic_read(&fscache_n_store_radix_deletes),
atomic_read(&fscache_n_store_pages_over_limit));
+ seq_printf(m, "VmScan : nos=%u gon=%u bsy=%u can=%u\n",
+ atomic_read(&fscache_n_store_vmscan_not_storing),
+ atomic_read(&fscache_n_store_vmscan_gone),
+ atomic_read(&fscache_n_store_vmscan_busy),
+ atomic_read(&fscache_n_store_vmscan_cancelled));
+
seq_printf(m, "Ops : pend=%u run=%u enq=%u can=%u rej=%u\n",
atomic_read(&fscache_n_op_pend),
atomic_read(&fscache_n_op_run),
diff --git a/fs/nfs/fscache.c b/fs/nfs/fscache.c
index 70fad69eb95..fa588006588 100644
--- a/fs/nfs/fscache.c
+++ b/fs/nfs/fscache.c
@@ -359,17 +359,13 @@ int nfs_fscache_release_page(struct page *page, gfp_t gfp)
BUG_ON(!cookie);
- if (fscache_check_page_write(cookie, page)) {
- if (!(gfp & __GFP_WAIT))
- return 0;
- fscache_wait_on_page_write(cookie, page);
- }
-
if (PageFsCache(page)) {
dfprintk(FSCACHE, "NFS: fscache releasepage (0x%p/0x%p/0x%p)\n",
cookie, page, nfsi);
- fscache_uncache_page(cookie, page);
+ if (!fscache_maybe_release_page(cookie, page, gfp))
+ return 0;
+
nfs_add_fscache_stats(page->mapping->host,
NFSIOS_FSCACHE_PAGES_UNCACHED, 1);
}