Re: [PATCH 1/4] migrate: Allow migrate_misplaced_folio APIs without a VMA

[Gregory Price <gourry@gourry.net>]

On Thu, Nov 28, 2024 at 07:12:11PM +0800, Huang, Ying wrote:
> Hi, Gregory,
> 
> Gregory Price <gourry@gourry.net> writes:
> 
> > To migrate unmapped pagecache folios, migrate_misplaced_folio and
> > migrate_misplaced_folio_prepare must handle folios without VMAs.
> 
> IMHO, it's better to use migrate_misplaced_folio() instead of
> migrate_misplaced_folio for readability in patch title and description.
> 
> > migrate_misplaced_folio_prepare checks VMA for exec bits, so allow
> > a NULL VMA when it does not have a mapping.
> >
> > migrate_misplaced_folio must call migrate_pages with MIGRATE_SYNC
> > when in the pagecache path because it is a synchronous context.
> 
> I don't find the corresponding implementation for this.  And, I don't
> think it's a good idea to change from MIGRATE_ASYNC to MIGRATE_SYNC.
> This may cause too long page access latency for page placement
> optimization.  The downside may offset the benefit.
> 
> And, it appears that we can delete the "vma" parameter of
> migrate_misplaced_folio() because it's not used now.  This is a trivial
> code cleanup.
>

This patch apparently got a bit away from me and was heavily reduced
from its initial form.  This commit message is just wrong now.  I will
update this and the 2nd commit and probably submit them separately.

~Gregory