[PATCH] memcg: remove incorrect underflow check

[Hugh Dickins <hughd@google.com>]

From: Greg Thelen <gthelen@google.com>

When a memcg is deleted mem_cgroup_reparent_charges() moves charged
memory to the parent memcg.  As of v3.11-9444-g3ea67d0 "memcg: add per
cgroup writeback pages accounting" there's bad pointer read.  The goal
was to check for counter underflow.  The counter is a per cpu counter
and there are two problems with the code:
(1) per cpu access function isn't used, instead a naked pointer is
    used which easily causes oops.
(2) the check doesn't sum all cpus

Test:
  $ cd /sys/fs/cgroup/memory
  $ mkdir x
  $ echo 3 > /proc/sys/vm/drop_caches
  $ (echo $BASHPID >> x/tasks && exec cat) &
  [1] 7154
  $ grep ^mapped x/memory.stat
  mapped_file 53248
  $ echo 7154 > tasks
  $ rmdir x
  <OOPS>

The fix is to remove the check.  It's currently dangerous and isn't
worth fixing it to use something expensive, such as
percpu_counter_sum(), for each reparented page.  __this_cpu_read()
isn't enough to fix this because there's no guarantees of the current
cpus count.  The only guarantees is that the sum of all per-cpu
counter is >= nr_pages.

Fixes: 3ea67d06e467 ("memcg: add per cgroup writeback pages accounting")
Reported-and-tested-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Greg Thelen <gthelen@google.com>
Reviewed-by: Sha Zhengju <handai.szj@taobao.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Hugh Dickins <hughd@google.com>
---

This patch fixes an actual crash (which I hit too this morning), so is
more important than the accounting fix on the next line which went in on
Wednesday.  It looks like this one got forgotten amidst discussion of the
others: I've updated Greg's original to apply to the current tree.

 mm/memcontrol.c | 1 -
 1 file changed, 1 deletion(-)

--- 3.12-rc7+/mm/memcontrol.c
+++ linux/mm/memcontrol.c
@@ -3782,7 +3782,6 @@ 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_sub(from->stat->count[idx], nr_pages);
 	__this_cpu_add(to->stat->count[idx], nr_pages);
 	preempt_enable();

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