Re: [syzbot] [mm?] WARNING in deferred_split_folio

David Hildenbrand (Arm)
2026-04-01T11:23:04Z

On 4/1/26 13:20, Lance Yang wrote:
>
> On Wed, Apr 01, 2026 at 01:00:13PM +0200, David Hildenbrand (Arm) wrote:
>> On 4/1/26 12:53, Lance Yang wrote:
>>>
>>> +Cc Deepanshu
>>>
>>>
>>> Afraid not, it closes the remove_migration_ptes() ->
>>> deferred_split_folio() race, but opens a new one with the shrinker, IIUC
>>>
>>> Once dst is on the deferred split queue, deferred_split_scan() can
>>> pick it up immediately. The shrinker unconditionally dequeues every
>>> folio it visits:
>>>
>>> list_del_init(&folio->_deferred_list); /* always */
>>>
>>> Then for a non-partially-mapped folio, if folio_trylock() fails
>>> (dst is still locked by migration), it falls through to:
>>>
>>> next:
>>> if (did_split || !folio_test_partially_mapped(folio))
>>> continue; /* not requeued, dst silently lost */
>>>
>>> so it is *not* requeued.
>>
>> How is that different to the shrinker just trying to lock the folio before we
>> unlock it and failing? The race already exists?
>
> Ouch, you're right, I was wrong - the trylock drop is a pre-existing
> issue, not caused by the reorder ;)
>
>>
>> To sort out that race a trylock must not result in the folio getting
>> discarded.
>
> Nice, LGTM!
>
> Given that the "trylock -> drop" behavior seems to exist already today,
> do you think it's worth fixing that together with the reorder?

I'd do it in a single shot if possible.

Can you craft something? (cc stable etc)

--
Cheers,

David