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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 <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Greg Thelen <gthelen@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
KAMEZAWA Hiroyuki 2010-10-27 15:33:39 -07:00 committed by Linus Torvalds
parent 45531757b4
commit 0c270f8f99
1 changed files with 2 additions and 1 deletions
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3
mm/memcontrol.c
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@ -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: