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

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Message-ID: <1ce8408d-32b0-469d-a560-2e0050cb91d2@lucifer.local>
Date: Thu, 26 Jun 2025 05:57:12 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com,
        baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
        npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] khugepaged: Reduce race probability between
 migration and khugepaged

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 :)