Re: [PATCH] mm: migrate: requeue destination folio on deferred split queue

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/6/26 17:15, Usama Arif wrote:
> 
> 
> On 06/03/2026 14:46, Zi Yan wrote:
>> On 6 Mar 2026, at 9:12, Usama Arif wrote:
>>
>>>
>>> Yes you are right. How about something like below? We also won't need to check
>>> for anon and non-device folios with this as we only set the the flag if it was
>>> already on deferred_split list.
>>
>> BTW, migrate_pages() tries to split partially mapped folios before migration[1],
>> so what remains in the deferred_list would be:
>>
>> 1. partially mapped but with a pin,
>> 2. fully mapped but potentially underused.
>>
> 
> Yes, thats right.
> 
>> I wonder if you want to do an underused scan before migration and try to split
>> underused THPs.
> 
> hmm, I think we should keep THPs as is if there is no memory pressure (proactive
> or otherwise). Scanning THPs for zeros has a cost and we would also lose the benefit
> of THPs when we dont need memory.
> 
>> Or to avoid this additional scan, find a way of detecting
>> zero pages at page copy time and split it after migration.
>>
> 
> Yeah but I think we lose the benefits of THPs after migration when we dont need
> additional memory?
> 
>> Anyway, it seems that all large folios are in this deferred_list. Maybe, like
>> David suggested in his LSFMM proposal, we should scan large folios on LRU lists
>> at reclaim time instead, since there is not much difference between deferred_list
>> and LRU lists right now.
>>
> 
> Yeah the THP shrinker is a very basic implementation and there are a lot of 
> 
>>
>> [1] https://elixir.bootlin.com/linux/v6.19.3/source/mm/migrate.c#L1840
>>
> 
> Also Johannes pointed out its not great storing this information in page flags,
> we can just keep it as local variable. This is what the patch would look like:

Much cleaner.

-- 
Cheers,

David