Re: [PATCH v2 1/4] mm/migrate: do not migrate folios mapped into VM_LOCKED VMAs under compaction

[Wandun <chenwandun1@gmail.com>]

On 7/7/26 21:55, Lorenzo Stoakes wrote:
> On Tue, Jul 07, 2026 at 02:44:50PM +0100, Lorenzo Stoakes wrote:
>> See above about deduplicating.
>>
>>> +			ttu |= TTU_RESPECT_MLOCK;
>>
>> Hmm. I don't love 'respect mlock'. I guess we only know about the reason
>> being compaction here.
>>
>> But I'm confused anyway. We have the folio, why aren't we just checking for
>> PG_mlocked() here instead of getting the rmap to see if it's mapped
>> anywhere with VMA_LOCKED_BIT?
> 
> Also, since compaction_allow_unevictable() is a function that is accessible
> elsewhere, you could literally just have a TTU_MIGRATION here instead and have
> the rmap logic call compaction_allow_unevictable() instead rather than this.
Do you mean:
1. change TTU_RESPECT_MLOCK to TTU_MIGRATION, that'is OK.
2. move call compaction_allow_unevictable() to try_to_migrate_one? but as you suggested
   migrate_mlock_allowed function, it already called compaction_allow_unevictable()
   in order to determine whether TTU_MIGRATION needs to be added. Did I misunderstand
   something somewhere?

> 
> And then you could adapt the function I suggested before not to take a reason
> parameter but rather a 'is_migration' one instead possibly and then pass (ttu &
> TTU_MIGRATION) in.
> 
> BUT. I still question whether this is at all needed since you have the folio you
> can check for PG_mlocked...

I described a race scenario at this link:
https://lore.kernel.org/lkml/c372e68f-2bfc-45b6-a431-01bf55717247@gmail.com/


> 
> Cheers, Lorenzo