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Subject[PATCH 4.4 34/54] mm/memcg: fix refcount error while moving and swapping
Date
From: Hugh Dickins <hughd@google.com>

commit 8d22a9351035ef2ff12ef163a1091b8b8cf1e49c upstream.

It was hard to keep a test running, moving tasks between memcgs with
move_charge_at_immigrate, while swapping: mem_cgroup_id_get_many()'s
refcount is discovered to be 0 (supposedly impossible), so it is then
forced to REFCOUNT_SATURATED, and after thousands of warnings in quick
succession, the test is at last put out of misery by being OOM killed.

This is because of the way moved_swap accounting was saved up until the
task move gets completed in __mem_cgroup_clear_mc(), deferred from when
mem_cgroup_move_swap_account() actually exchanged old and new ids.
Concurrent activity can free up swap quicker than the task is scanned,
bringing id refcount down 0 (which should only be possible when
offlining).

Just skip that optimization: do that part of the accounting immediately.

Fixes: 615d66c37c75 ("mm: memcontrol: fix memcg id ref counter on swap charge move")
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2007071431050.4726@eggly.anvils
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/memcontrol.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4889,7 +4889,6 @@ static void __mem_cgroup_clear_mc(void)
if (!mem_cgroup_is_root(mc.to))
page_counter_uncharge(&mc.to->memory, mc.moved_swap);

- mem_cgroup_id_get_many(mc.to, mc.moved_swap);
css_put_many(&mc.to->css, mc.moved_swap);

mc.moved_swap = 0;
@@ -5067,7 +5066,8 @@ put: /* get_mctgt_type() gets the page
ent = target.ent;
if (!mem_cgroup_move_swap_account(ent, mc.from, mc.to)) {
mc.precharge--;
- /* we fixup refcnts and charges later. */
+ mem_cgroup_id_get_many(mc.to, 1);
+ /* we fixup other refcnts and charges later. */
mc.moved_swap++;
}
break;

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