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.

Fixes: 1908792a368b ("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>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c75c4face02e6f09fedfed4eee2317eb4d5cc72d..13f559af1ab6a359b40c8641be754be7fd56079a 100644 (file)
--- a/mm/memcontrol.c
+++ b/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;