Messages in this thread Patch in this message | | | Date | Wed, 29 Feb 2012 18:44:59 -0800 (PST) | From | Hugh Dickins <> | Subject | [PATCH v2 next] memcg: fix deadlock by avoiding stat lock when anon |
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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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