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

[Zi Yan]

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@xxxxxxx>
> ---
>
> 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@xxxxxxx/
>
>  mm/khugepaged.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

Best Regards,
Yan, Zi