I think this small race is not very critical but it's bug.
We have this race since 2.6.34.
=
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Now. 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 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 specail care.
To handle this, at claering 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@xxxxxxxxxxxxxx>
---
mm/memcontrol.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
Index: lockless-update/mm/memcontrol.c
===================================================================
--- lockless-update.orig/mm/memcontrol.c
+++ lockless-update/mm/memcontrol.c
@@ -1485,7 +1485,8 @@ void mem_cgroup_update_file_mapped(struc
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:
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