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

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Message-ID: <1B1A99C7-035B-423B-A389-7BB43D858439@nvidia.com>
Date: Thu, 03 Jul 2025 10:07:55 -0400
From: Zi Yan <ziy@...dia.com>
To: Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, david@...hat.com,
 baolin.wang@...ux.alibaba.com, lorenzo.stoakes@...cle.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] khugepaged: Reduce race probability between migration
 and khugepaged

On 3 Jul 2025, at 1:48, 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.
>
> Note that, this is not a "fix" since it only reduces the chance of
> interference of khugepaged with migration, wherein both the kernel
> functionalities are deemed "best-effort".
>
> Signed-off-by: Dev Jain <dev.jain@....com>
> ---
>
> v1->v2:
>  - Remove SCAN_PMD_MIGRATION, merge into SCAN_PMD_MAPPED (David, Anshuman)
>  - Add a comment (Lorenzo)
>
> v1:
>  - https://lore.kernel.org/all/20250630044837.4675-1-dev.jain@arm.com/
>
>  mm/khugepaged.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>

Reviewed-by: Zi Yan <ziy@...dia.com>

Best Regards,
Yan, Zi