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Wed, 02 Sep 2026 11:34:38 +0000 X-Mizu-Trace-ID: 0649d5faa369876e X-Migadu-Flow: FLOW_OUT Message-ID: <5753d853-df04-4918-9852-c23842c66428@linux.dev> Date: Wed, 2 Sep 2026 12:34:38 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/shrinker: fix bogus set_shrinker_bit() with cgroup.memory=nokmem To: Jiayuan Chen , linux-mm@kvack.org Cc: Andrew Morton , Dave Chinner , Qi Zheng , Roman Gushchin , Muchun Song , Shakeel Butt , Kairui Song , Johannes Weiner , linux-kernel@vger.kernel.org References: <20260902073800.305481-1-jiayuan.chen@linux.dev> Content-Language: en-US From: Usama Arif In-Reply-To: <20260902073800.305481-1-jiayuan.chen@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: F0E9E40002 X-Stat-Signature: hn5zmwuqqqp8a85fg3qkgw7ihsgb8dgk X-Rspam-User: X-HE-Tag: 1788348879-464133 X-HE-Meta: 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 KEws5/8k KiSBf2QnAuKdqc/YkKIpQAd1ZeX0PUMGb4mzOhPeJBMM5ppZSJbYtASBE8czArAJlOmUhOXSnZo7ZKfQBejERn3oJeOEqwRcBDHApcBqlGOiRv0NaKLfyEKKg/lqTixMTqjTrRb5aHpiHYvPkF8XftHFWHjVMn6lkDWPPuU2IB3ZAXTORnB5NqDaCMjRtxltjvacFXR2Em2pd4/QSoFNx1E4aZvOPUDLl68Bxwh3Vy6yR/2pa8tjgOfMD7bi/mY8IhZrPtRalwJ+FUoc8OLwLcwIhzHUwm3jNKxjPQF/JjkrkGN/r2ghdim3ycyS/d3XWLVWo Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 02/09/2026 08:37, Jiayuan Chen wrote: > 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. > > Fixes: fafaeceb89a5 ("mm: switch deferred split shrinker to list_lru") > Signed-off-by: Jiayuan Chen > --- > 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)) Thanks for the fix! I think the initialization would look better in shrinker_alloc()?