linux-kernel - [PATCH 5.4 123/138] mm/memcg: fix refcount error while moving and swapping

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Date:   Mon, 27 Jul 2020 16:05:18 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 5.4 123/138] mm/memcg: fix refcount error while moving and swapping

From: Hugh Dickins <hughd@...gle.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@...gle.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Reviewed-by: Alex Shi <alex.shi@...ux.alibaba.com>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Alex Shi <alex.shi@...ux.alibaba.com>
Cc: Shakeel Butt <shakeelb@...gle.com>
Cc: Michal Hocko <mhocko@...e.com>
Cc: <stable@...r.kernel.org>
Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2007071431050.4726@eggly.anvils
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

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

--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5770,7 +5770,6 @@ 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;
@@ -5961,7 +5960,8 @@ 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;