From 932cfb25e7ce98d1f93895671ec186a3087e4f80 Mon Sep 17 00:00:00 2001 From: Jiayuan Chen Date: Wed, 2 Sep 2026 15:37:59 +0800 Subject: [PATCH] mm/shrinker: fix bogus set_shrinker_bit() with cgroup.memory=nokmem With cgroup.memory=nokmem, shrinker_memcg_alloc() bails out early and never allocates an id, so shrinker->id keeps the 0 it got from the kzalloc() in shrinker_alloc(). __list_lru_init() then copies that 0 into lru->shrinker_id, where it looks like a valid bit index. Nothing calls expand_shrinker_info() on nokmem either, so shrinker_nr_max stays 0 and every memcg ends up with an empty map (map_nr_max == 0). deferred_split_folio() hands a real memcg to __list_lru_add() regardless of whether the lru is memcg aware, so the first THP queued in a cgroup does set_shrinker_bit(memcg, nid, 0) and trips the bounds check: WARNING: mm/shrinker.c:212 at set_shrinker_bit+0x7d/0x90, CPU#126 Call Trace: deferred_split_folio+0x18c/0x220 map_anon_folio_pmd_nopf+0xdd/0x130 map_anon_folio_pmd_pf+0x14/0xb0 do_huge_pmd_anonymous_page+0x1a1/0x620 __handle_mm_fault+0xea9/0x10d0 handle_mm_fault+0xe5/0x320 do_user_addr_fault+0x1cc/0x870 exc_page_fault+0x81/0x1b0 asm_exc_page_fault+0x27/0x30 Harmless, the WARN_ON_ONCE() is what keeps the out of bounds unit[] read from happening, but the id should not look valid in the first place. Clear it before returning. Two other spots could paper over this: drop the id in __list_lru_init() when nokmem turns memcg_aware off, or make deferred_split_folio() pass NULL like list_lru_add_obj() does. Both leave shrinker->id lying around for the next caller, so fix it where the id is handed out. Link: https://lore.kernel.org/20260902073800.305481-1-jiayuan.chen@linux.dev Fixes: fafaeceb89a5 ("mm: switch deferred split shrinker to list_lru") Signed-off-by: Jiayuan Chen Acked-by: Shakeel Butt Cc: Usama Arif Cc: Dave Chinner Cc: Johannes Weiner Cc: Kairui Song Cc: Muchun Song Cc: Roman Gushchin Cc: Signed-off-by: Andrew Morton --- mm/shrinker.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/mm/shrinker.c b/mm/shrinker.c index a70aab124a0e..7ec2a9704f6f 100644 --- a/mm/shrinker.c +++ b/mm/shrinker.c @@ -227,6 +227,8 @@ static int shrinker_memcg_alloc(struct shrinker *shrinker) { int id; + shrinker->id = -1; + if (mem_cgroup_disabled()) return -ENOSYS; if (mem_cgroup_kmem_disabled() && !(shrinker->flags & SHRINKER_NONSLAB))