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| author | KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> | 2010-10-28 02:33:39 +0400 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2010-10-28 05:03:09 +0400 |
| commit | 0c270f8f9988fb0d93ea214fdcff7ab90eb3d894 (patch) | |
| tree | b4f77a6714d245841ecec535ccbeba535c0d29a4 /mm | |
| parent | 45531757b45cae0ce64c5aff08c2534d5a0fa3e7 (diff) | |
| download | linux-0c270f8f9988fb0d93ea214fdcff7ab90eb3d894.tar.xz | |
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>
Diffstat (limited to 'mm')
| -rw-r--r-- | mm/memcontrol.c | 3 |
1 files changed, 2 insertions, 1 deletions
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: |