[merged mm-stable] mm-list_lru-dont-copy-stale-shrinker-id-from-non-memcg-aware-shrinkers.patch removed from -mm tree

[merged mm-stable] mm-list_lru-dont-copy-stale-shrinker-id-from-non-memcg-aware-shrinkers.patch removed from -mm tree

[Andrew Morton]

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The quilt patch titled
     Subject: mm/list_lru: don't copy stale shrinker id from non-memcg-aware shrinkers
has been removed from the -mm tree.  Its filename was
     mm-list_lru-dont-copy-stale-shrinker-id-from-non-memcg-aware-shrinkers.patch

This patch was dropped because it was merged into the mm-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Qinyun Tan <qinyuntan@linux.alibaba.com>
Subject: mm/list_lru: don't copy stale shrinker id from non-memcg-aware shrinkers
Date: Tue, 1 Sep 2026 19:51:04 +0800

With cgroup.memory=nokmem, shrinker_memcg_alloc() fails with -ENOSYS for
shrinkers without SHRINKER_NONSLAB, and shrinker_alloc() falls back to a
non-memcg-aware shrinker.  On this fallback path, shrinker->id is never
assigned and keeps 0 from kzalloc(), which is a valid id belonging to
whichever memcg-aware shrinker registers first.

__list_lru_init() copies shrinker->id unconditionally, so every list_lru
backed by such a fallback shrinker (thp-deferred_split, zswap-shrinker,
workingset shadow nodes, superblock lrus, ...) ends up with
lru->shrinker_id == 0 instead of -1.

Under nokmem the list_lru collapses to the shared per-node lists, but
__list_lru_add() still calls set_shrinker_bit() against the memcg of
the added object.  Most list_lru users are unaffected because their
objects resolve to a NULL memcg without kmem accounting, but the THP
deferred split queue holds user folios, which are charged regardless of
nokmem.  On a system where no SHRINKER_NONSLAB shrinker registers,
shrinker_nr_max stays 0 and every memcg's shrinker_info has map_nr_max
== 0, so the first folio added by khugepaged triggers on every boot:

  WARNING: mm/shrinker.c:212 at set_shrinker_bit+0x99/0xa0

On systems where a SHRINKER_NONSLAB shrinker (btrfs, xfs) did register and
expand the maps, there is no warning; instead bit 0 is set spuriously for
an unrelated shrinker.

shrinker->id is only meaningful while SHRINKER_MEMCG_AWARE is set, and all
readers inside mm/shrinker.c already check the flag before using the id. 
Make __list_lru_init() do the same and fall back to -1, so
set_shrinker_bit() is never reached with a bogus id.  The stale
shrinker->id itself is left as is; cleaning that up is a separate topic.

Verified on a machine booting with cgroup.memory=nokmem and
CONFIG_TRANSPARENT_HUGEPAGE=y: the warning fires once per boot from
khugepaged, disappears when nokmem is removed from the command line, and
no longer triggers with this fix applied and nokmem set.

Link: https://lore.kernel.org/20260901115104.2944996-1-qinyuntan@linux.alibaba.com
Fixes: fafaeceb89a5e ("mm: switch deferred split shrinker to list_lru")
Signed-off-by: Qinyun Tan <qinyuntan@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reported-by: Wentao Guan <guanwentao@uniontech.com>
Closes: https://lore.kernel.org/linux-mm/20260904190028.21542-1-guanwentao@uniontech.com/
Tested-by: Wentao Guan <guanwentao@uniontech.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Dave Chinner <david@fromorbit.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Xunlei Pang <xlpang@linux.alibaba.com>
Cc: <stable@vger.kernel.org>
---

 mm/list_lru.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/mm/list_lru.c~mm-list_lru-dont-copy-stale-shrinker-id-from-non-memcg-aware-shrinkers
+++ a/mm/list_lru.c
@@ -666,7 +666,12 @@ int __list_lru_init(struct list_lru *lru
 	int i;
 
 #ifdef CONFIG_MEMCG
-	if (shrinker)
+	/*
+	 * If the shrinker fell back to being non-memcg-aware (e.g. with
+	 * cgroup.memory=nokmem), its id was never assigned and holds a
+	 * stale 0. Don't let set_shrinker_bit() act on it.
+	 */
+	if (shrinker && (shrinker->flags & SHRINKER_MEMCG_AWARE))
 		lru->shrinker_id = shrinker->id;
 	else
 		lru->shrinker_id = -1;
_

Patches currently in -mm which might be from qinyuntan@linux.alibaba.com are