+ mm-memcg-fix-refcount-error-while-moving-and-swapping.patch added to -mm tree

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/memcg: fix refcount error while moving and swapping
has been added to the -mm tree.  Its filename is
     mm-memcg-fix-refcount-error-while-moving-and-swapping.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/mm-memcg-fix-refcount-error-while-moving-and-swapping.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/mm-memcg-fix-refcount-error-while-moving-and-swapping.patch

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------------------------------------------------------
From: Hugh Dickins <hughd@google.com>
Subject: mm/memcg: fix refcount error while moving and swapping

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.

Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2007071431050.4726@eggly.anvils
Fixes: 615d66c37c75 ("mm: memcontrol: fix memcg id ref counter on swap charge move")
Signed-off-by: Hugh Dickins <hughd@google.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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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

--- a/mm/memcontrol.c~mm-memcg-fix-refcount-error-while-moving-and-swapping
+++ a/mm/memcontrol.c
@@ -5669,7 +5669,6 @@ 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;
@@ -5860,7 +5859,8 @@ 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;
_

Patches currently in -mm which might be from hughd@google.com are

mm-memcg-fix-refcount-error-while-moving-and-swapping.patch