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authorHugh Dickins <hughd@google.com>2012-07-11 14:02:47 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2012-07-11 16:04:48 -0700
commitd189922862e03ce6c7adc1e99d3b94e632dc8e89 (patch)
parentf21f8062201fc6361f65de92e758a76375ba8c59 (diff)
shmem: fix negative rss in memcg memory.stat
When adding the page_private checks before calling shmem_replace_page(), I did realize that there is a further race, but thought it too unlikely to need a hurried fix. But independently I've been chasing why a mem cgroup's memory.stat sometimes shows negative rss after all tasks have gone: I expected it to be a stats gathering bug, but actually it's shmem swapping's fault. It's an old surprise, that when you lock_page(lookup_swap_cache(swap)), the page may have been removed from swapcache before getting the lock; or it may have been freed and reused and be back in swapcache; and it can even be using the same swap location as before (page_private same). The swapoff case is already secure against this (swap cannot be reused until the whole area has been swapped off, and a new swapped on); and shmem_getpage_gfp() is protected by shmem_add_to_page_cache()'s check for the expected radix_tree entry - but a little too late. By that time, we might have already decided to shmem_replace_page(): I don't know of a problem from that, but I'd feel more at ease not to do so spuriously. And we have already done mem_cgroup_cache_charge(), on perhaps the wrong mem cgroup: and this charge is not then undone on the error path, because PageSwapCache ends up preventing that. It's this last case which causes the occasional negative rss in memory.stat: the page is charged here as cache, but (sometimes) found to be anon when eventually it's uncharged - and in between, it's an undeserved charge on the wrong memcg. Fix this by adding an earlier check on the radix_tree entry: it's inelegant to descend the tree twice, but swapping is not the fast path, and a better solution would need a pair (try+commit) of memcg calls, and a rework of shmem_replace_page() to keep out of the swapcache. We can use the added shmem_confirm_swap() function to replace the find_get_page+page_cache_release we were already doing on the error path. And add a comment on that -EEXIST: it seems a peculiar errno to be using, but originates from its use in radix_tree_insert(). [It can be surprising to see positive rss left in a memcg's memory.stat after all tasks have gone, since it is supposed to count anonymous but not shmem. Aside from sharing anon pages via fork with a task in some other memcg, it often happens after swapping: because a swap page can't be freed while under writeback, nor while locked. So it's not an error, and these residual pages are easily freed once pressure demands.] Signed-off-by: Hugh Dickins <hughd@google.com> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Cc: Michal Hocko <mhocko@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 files changed, 29 insertions, 12 deletions
diff --git a/mm/shmem.c b/mm/shmem.c
index 3f696f7d9bac..294364a24a1f 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -264,6 +264,24 @@ static int shmem_radix_tree_replace(struct address_space *mapping,
+ * Sometimes, before we decide whether to proceed or to fail, we must check
+ * that an entry was not already brought back from swap by a racing thread.
+ *
+ * Checking page is not enough: by the time a SwapCache page is locked, it
+ * might be reused, and again be SwapCache, using the same swap as before.
+ */
+static bool shmem_confirm_swap(struct address_space *mapping,
+ pgoff_t index, swp_entry_t swap)
+ void *item;
+ rcu_read_lock();
+ item = radix_tree_lookup(&mapping->page_tree, index);
+ rcu_read_unlock();
+ return item == swp_to_radix_entry(swap);
* Like add_to_page_cache_locked, but error if expected item has gone.
static int shmem_add_to_page_cache(struct page *page,
@@ -1124,9 +1142,9 @@ repeat:
/* We have to do this with page locked to prevent races */
if (!PageSwapCache(page) || page_private(page) != swap.val ||
- page->mapping) {
+ !shmem_confirm_swap(mapping, index, swap)) {
error = -EEXIST; /* try again */
- goto failed;
+ goto unlock;
if (!PageUptodate(page)) {
error = -EIO;
@@ -1142,9 +1160,12 @@ repeat:
error = mem_cgroup_cache_charge(page, current->mm,
- if (!error)
+ if (!error) {
error = shmem_add_to_page_cache(page, mapping, index,
gfp, swp_to_radix_entry(swap));
+ /* We already confirmed swap, and make no allocation */
+ VM_BUG_ON(error);
+ }
if (error)
goto failed;
@@ -1245,14 +1266,10 @@ decused:
shmem_unacct_blocks(info->flags, 1);
- if (swap.val && error != -EINVAL) {
- struct page *test = find_get_page(mapping, index);
- if (test && !radix_tree_exceptional_entry(test))
- page_cache_release(test);
- /* Have another try if the entry has changed */
- if (test != swp_to_radix_entry(swap))
- error = -EEXIST;
- }
+ if (swap.val && error != -EINVAL &&
+ !shmem_confirm_swap(mapping, index, swap))
+ error = -EEXIST;
if (page) {
@@ -1264,7 +1281,7 @@ failed:
goto repeat;
- if (error == -EEXIST)
+ if (error == -EEXIST) /* from above or from radix_tree_insert */
goto repeat;
return error;