From d189922862e03ce6c7adc1e99d3b94e632dc8e89 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Wed, 11 Jul 2012 14:02:47 -0700 Subject: [PATCH] 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 Acked-by: Johannes Weiner Cc: KAMEZAWA Hiroyuki Cc: Michal Hocko Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/shmem.c | 41 +++++++++++++++++++++++++++++------------ 1 file 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 @@ -263,6 +263,24 @@ static int shmem_radix_tree_replace(struct address_space *mapping, return 0; } +/* + * 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. */ @@ -1124,9 +1142,9 @@ repeat: /* We have to do this with page locked to prevent races */ lock_page(page); 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, gfp & GFP_RECLAIM_MASK); - 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: unacct: shmem_unacct_blocks(info->flags, 1); failed: - 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; +unlock: if (page) { unlock_page(page); page_cache_release(page); @@ -1264,7 +1281,7 @@ failed: spin_unlock(&info->lock); goto repeat; } - if (error == -EEXIST) + if (error == -EEXIST) /* from above or from radix_tree_insert */ goto repeat; return error; } -- 2.20.1