Re: [PATCH] mm/shrinker: fix bogus set_shrinker_bit() with cgroup.memory=nokmem

[Jiayuan Chen <jiayuan.chen@linux.dev>]

on 9/2/26 7:34 PM, Usama Arif wrote:
>
> 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:
>>   <TASK>
>>   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
>>   </TASK>
>>
>> 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 <jiayuan.chen@linux.dev>
>> ---
>>   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()?


Well, we have to add macro if we move it to shrinker_alloc()


     shrinker->flags = flags | SHRINKER_ALLOCATED;
     shrinker->seeks = DEFAULT_SEEKS;

+#ifdef CONFIG_MEMCG
+        shrinker->id = -1;
+#endif


No strong preference for me :)