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

[Lorenzo Stoakes]

On Thu, Jul 09, 2026 at 04:25:27PM +0800, Wandun wrote:
>
>
> 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?

The next paragraph addresses this.

>
> >
> > 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@xxxxxxxxx/

OK thanks. You really have to spell this out in the code in a comment, this is
really non-obvious.

I will try to look at your reply in due course (am between jobs atm).

>
>
> >
> > Cheers, Lorenzo
>

Thanks, Lorenzo