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authorGreg Thelen <gthelen@google.com>2013-10-30 13:56:21 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2013-10-30 14:27:03 -0700
commit5e8cfc3c75b3e43497389896c0ecda62fc311ce9 (patch)
treee78fc7110c7e627b22e09a6458a2395ad128858b
parentbd09d9a35111b6ffc0c7585d3853d0ec7f9f1eb4 (diff)
downloadlinux-5e8cfc3c75b3e43497389896c0ecda62fc311ce9.tar.gz
memcg: use __this_cpu_sub() to dec stats to avoid incorrect subtrahend casting
As of commit 3ea67d06e467 ("memcg: add per cgroup writeback pages accounting") memcg counter errors are possible when moving charged memory to a different memcg. Charge movement occurs when processing writes to memory.force_empty, moving tasks to a memcg with memcg.move_charge_at_immigrate=1, or memcg deletion. An example showing error after memory.force_empty: $ cd /sys/fs/cgroup/memory $ mkdir x $ rm /data/tmp/file $ (echo $BASHPID >> x/tasks && exec mmap_writer /data/tmp/file 1M) & [1] 13600 $ grep ^mapped x/memory.stat mapped_file 1048576 $ echo 13600 > tasks $ echo 1 > x/memory.force_empty $ grep ^mapped x/memory.stat mapped_file 4503599627370496 mapped_file should end with 0. 4503599627370496 == 0x10,0000,0000,0000 == 0x100,0000,0000 pages 1048576 == 0x10,0000 == 0x100 pages This issue only affects the source memcg on 64 bit machines; the destination memcg counters are correct. So the rmdir case is not too important because such counters are soon disappearing with the entire memcg. But the memcg.force_empty and memory.move_charge_at_immigrate=1 cases are larger problems as the bogus counters are visible for the (possibly long) remaining life of the source memcg. The problem is due to memcg use of __this_cpu_from(.., -nr_pages), which is subtly wrong because it subtracts the unsigned int nr_pages (either -1 or -512 for THP) from a signed long percpu counter. When nr_pages=-1, -nr_pages=0xffffffff. On 64 bit machines stat->count[idx] is signed 64 bit. So memcg's attempt to simply decrement a count (e.g. from 1 to 0) boils down to: long count = 1 unsigned int nr_pages = 1 count += -nr_pages /* -nr_pages == 0xffff,ffff */ count is now 0x1,0000,0000 instead of 0 The fix is to subtract the unsigned page count rather than adding its negation. This only works once "percpu: fix this_cpu_sub() subtrahend casting for unsigneds" is applied to fix this_cpu_sub(). Signed-off-by: Greg Thelen <gthelen@google.com> Acked-by: Tejun Heo <tj@kernel.org> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--mm/memcontrol.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 34d3ca9572d..497ec33ff22 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3774,7 +3774,7 @@ void mem_cgroup_move_account_page_stat(struct mem_cgroup *from,
/* Update stat data for mem_cgroup */
preempt_disable();
WARN_ON_ONCE(from->stat->count[idx] < nr_pages);
- __this_cpu_add(from->stat->count[idx], -nr_pages);
+ __this_cpu_sub(from->stat->count[idx], nr_pages);
__this_cpu_add(to->stat->count[idx], nr_pages);
preempt_enable();
}