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mm/memcg: fix refcount error while moving and swapping

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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: 615d66c37c ("mm: memcontrol: fix memcg id ref counter on swap charge move")
Signed-off-by: Hugh Dickins <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Alex Shi <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Alex Shi <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
This commit is contained in:
Hugh Dickins 2020-07-23 21:15:24 -07:00 • committed by Greg Kroah-Hartman
commit db949f60d9
1 changed files with 2 additions and 2 deletions
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4
mm/memcontrol.c
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@ -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;