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Subject[PATCH v2 next] memcg: fix deadlock by avoiding stat lock when anon
Fix deadlock in "memcg: use new logic for page stat accounting".

page_remove_rmap() first calls mem_cgroup_begin_update_page_stat(),
which may take move_lock_mem_cgroup(), unlocked at the end of
page_remove_rmap() by mem_cgroup_end_update_page_stat().

The PageAnon case never needs to mem_cgroup_dec_page_stat(page,
MEMCG_NR_FILE_MAPPED); but it often needs to mem_cgroup_uncharge_page(),
which does lock_page_cgroup(), while holding that move_lock_mem_cgroup().
Whereas mem_cgroup_move_account() calls move_lock_mem_cgroup() while
holding lock_page_cgroup().

Since mem_cgroup_begin and end are unnecessary here for PageAnon,
simply avoid the deadlock and wasted calls in that case.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
v2: added comment in the code so it's not thought just an optimization.
mm/rmap.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
--- 3.3-rc5-next/mm/rmap.c 2012-02-26 23:51:46.506050210 -0800
+++ linux/mm/rmap.c 2012-02-29 17:55:42.868665736 -0800
@@ -1166,10 +1166,18 @@ void page_add_file_rmap(struct page *pag
*/
void page_remove_rmap(struct page *page)
{
+ bool anon = PageAnon(page);
bool locked;
unsigned long flags;

- mem_cgroup_begin_update_page_stat(page, &locked, &flags);
+ /*
+ * The anon case has no mem_cgroup page_stat to update; but may
+ * uncharge_page() below, where the lock ordering can deadlock if
+ * we hold the lock against page_stat move: so avoid it on anon.
+ */
+ if (!anon)
+ mem_cgroup_begin_update_page_stat(page, &locked, &flags);
+
/* page still mapped by someone else? */
if (!atomic_add_negative(-1, &page->_mapcount))
goto out;
@@ -1181,7 +1189,7 @@ void page_remove_rmap(struct page *page)
* not if it's in swapcache - there might be another pte slot
* containing the swap entry, but page not yet written to swap.
*/
- if ((!PageAnon(page) || PageSwapCache(page)) &&
+ if ((!anon || PageSwapCache(page)) &&
page_test_and_clear_dirty(page_to_pfn(page), 1))
set_page_dirty(page);
/*
@@ -1190,7 +1198,7 @@ void page_remove_rmap(struct page *page)
*/
if (unlikely(PageHuge(page)))
goto out;
- if (PageAnon(page)) {
+ if (anon) {
mem_cgroup_uncharge_page(page);
if (!PageTransHuge(page))
__dec_zone_page_state(page, NR_ANON_PAGES);
@@ -1211,7 +1219,8 @@ void page_remove_rmap(struct page *page)
* faster for those pages still in swapcache.
*/
out:
- mem_cgroup_end_update_page_stat(page, &locked, &flags);
+ if (!anon)
+ mem_cgroup_end_update_page_stat(page, &locked, &flags);
}

/*

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