The patch titled
Subject: mm/list_lru: fix false warning of negative counter
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-list_lru-fix-false-warning-of-negative-counter.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-list_lru-fix-false-warning-of-negative-counter.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Kairui Song <kasong@xxxxxxxxxxx>
Subject: mm/list_lru: fix false warning of negative counter
Date: Mon, 23 Dec 2024 23:09:07 +0800
commit 2788cf0c401c ("memcg: reparent list_lrus and free kmemcg_id on css
offline") removed sanity checks for the nr_items counter's value because
it implemented list_lru re-parenting in a way that will redirect
children's list_lru to the parent before re-parenting the items in
list_lru. This will make item counter uncharging happen in the parent
while the item is still being held by the child. As a result, the
parent's counter value may become negative. This is acceptable because
re-parenting will sum up the children's counter values, and the parent's
counter will be fixed.
Later commit fb56fdf8b9a2 ("mm/list_lru: split the lock to per-cgroup
scope") reworked the re-parenting process, and removed the redirect. So
it added the sanity check back, assuming that as long as items are still
in the children's list_lru, parent's counter will not be uncharged.
But that assumption is incorrect. The xas_store in
memcg_reparent_list_lrus will set children's list_lru to NULL before
re-parenting the items, it redirects list_lru helpers to use parent's
list_lru just like before. But still, it's not a problem as re-parenting
will fix the counter.
Therefore, remove this sanity check, but add a new check to ensure that
the counter won't go negative in a different way: the child's list_lru
being re-parented should never have a negative counter, since re-parenting
should occur in order and fixes counters.
Link: https://lkml.kernel.org/r/20241223150907.1591-1-ryncsn@xxxxxxxxx
Fixes: fb56fdf8b9a2 ("mm/list_lru: split the lock to per-cgroup scope")
Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/Z2Bz9t92Be9l1xqj@lappy/
Cc: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Sasha Levin <sashal@xxxxxxxxxx>
Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/list_lru.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/mm/list_lru.c~mm-list_lru-fix-false-warning-of-negative-counter
+++ a/mm/list_lru.c
@@ -77,7 +77,6 @@ again:
spin_lock(&l->lock);
nr_items = READ_ONCE(l->nr_items);
if (likely(nr_items != LONG_MIN)) {
- WARN_ON(nr_items < 0);
rcu_read_unlock();
return l;
}
@@ -450,6 +449,7 @@ static void memcg_reparent_list_lru_one(
list_splice_init(&src->list, &dst->list);
if (src->nr_items) {
+ WARN_ON(src->nr_items < 0);
dst->nr_items += src->nr_items;
set_shrinker_bit(dst_memcg, nid, lru_shrinker_id(lru));
}
_
Patches currently in -mm which might be from kasong@xxxxxxxxxxx are
mm-list_lru-fix-false-warning-of-negative-counter.patch
mm-memcontrol-avoid-duplicated-memcg-enable-check.patch
mm-swap_cgroup-remove-swap_cgroup_cmpxchg.patch
mm-swap_cgroup-remove-global-swap-cgroup-lock.patch
mm-swap_cgroup-decouple-swap-cgroup-recording-and-clearing.patch
