Re: [PATCH v2 3/3] khugepaged: Reduce race probability between migration and khugepaged

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Jun 26, 2025 at 10:29:22AM +0530, Dev Jain wrote:
>
> On 26/06/25 10:27 am, Lorenzo Stoakes wrote:
> > On Thu, Jun 26, 2025 at 09:22:28AM +0530, Dev Jain wrote:
> > > On 25/06/25 6:58 pm, Lorenzo Stoakes wrote:
> > > > On Wed, Jun 25, 2025 at 11:28:06AM +0530, Dev Jain wrote:
> > > > > Suppose a folio is under migration, and khugepaged is also trying to
> > > > > collapse it. collapse_pte_mapped_thp() will retrieve the folio from the
> > > > > page cache via filemap_lock_folio(), thus taking a reference on the folio
> > > > > and sleeping on the folio lock, since the lock is held by the migration
> > > > > path. Migration will then fail in
> > > > > __folio_migrate_mapping -> folio_ref_freeze. Reduce the probability of
> > > > > such a race happening (leading to migration failure) by bailing out
> > > > > if we detect a PMD is marked with a migration entry.
> > > > >
> > > > > This fixes the migration-shared-anon-thp testcase failure on Apple M3.
> > > > Hm is this related to the series at all? Seems somewhat unrelated?
> > > Not related.
> > >
> > > > Is there a Fixes, Closes, etc.? Do we need something in stable?
> > > We don't need anything. This is an "expected race" in the sense that
> > > both migration and khugepaged collapse are best effort algorithms.
> > > I am just seeing a test failure on my system because my system hits
> > > the race more often. So this patch reduces the window for the race.
> > Does it rely on previous patches? If not probably best to send this one
> > separately :)
>
> To prevent rebasing headaches for others (if any) I thought to send all together.
> I'll send it separately if still that is the preference.
>
>

Oh actually would it be a pain to rebase given the previous 2 patches? Maybe
leave it then. And I can actually finally review it... :)