From 0c270f8f9988fb0d93ea214fdcff7ab90eb3d894 Mon Sep 17 00:00:00 2001 From: KAMEZAWA Hiroyuki Date: Wed, 27 Oct 2010 15:33:39 -0700 Subject: [PATCH] memcg: fix race in file_mapped accouting flag management Presently memory cgroup accounts file-mapped by counter and flag. counter is working in the same way with zone_stat but FileMapped flag only exists in memcg (for helping move_account). This flag can be updated wrongly in a case. Assume CPU0 and CPU1 and a thread mapping a page on CPU0, another thread unmapping it on CPU1. CPU0 CPU1 rmv rmap (mapcount 1->0) add rmap (mapcount 0->1) lock_page_cgroup() memcg counter+1 (some delay) set MAPPED FLAG. unlock_page_cgroup() lock_page_cgroup() memcg counter-1 clear MAPPED flag In the above sequence counter is properly updated but FLAG is not. This means that representing a state by a flag which is maintained by counter needs some special care. To handle this, when clearing a flag, this patch check mapcount directly and clear the flag only when mapcount == 0. (if mapcount >0, someone will make it to zero later and flag will be cleared.) Reverse case, dec-after-inc cannot be a problem because page_table_lock() works well for it. (IOW, to make above sequence, 2 processes should touch the same page at once with map/unmap.) Signed-off-by: KAMEZAWA Hiroyuki Cc: Balbir Singh Cc: Daisuke Nishimura Cc: Greg Thelen Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memcontrol.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 9be3cf8a5da4..0e3fdbd809c7 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -1485,7 +1485,8 @@ void mem_cgroup_update_file_mapped(struct page *page, int val) SetPageCgroupFileMapped(pc); } else { __this_cpu_dec(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]); - ClearPageCgroupFileMapped(pc); + if (!page_mapped(page)) /* for race between dec->inc counter */ + ClearPageCgroupFileMapped(pc); } done: