Re: [PATCH v3] khugepaged: Reduce race probability between migration and khugepaged

[Nico Pache <npache@redhat.com>]

On Thu, Jul 3, 2025 at 10:04 PM Dev Jain <dev.jain@arm.com> 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".
>
> Acked-by: David Hildenbrand <david@redhat.com>
> Acked-by: Oscar Salvador <osalvador@suse.de>
> Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Signed-off-by: Dev Jain <dev.jain@arm.com>

LGTM! This is a nice check to have here :)

Reviewed-by: Nico Pache <npache@redhat.com>
> ---
>
> v2->v3:
>  - Improve comment (David)
>
> 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 | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 1aa7ca67c756..a55fb1dcd224 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -941,6 +941,14 @@ static inline int check_pmd_state(pmd_t *pmd)
>
>         if (pmd_none(pmde))
>                 return SCAN_PMD_NONE;
> +
> +       /*
> +        * The folio may be under migration when khugepaged is trying to
> +        * collapse it. Migration success or failure will eventually end
> +        * up with a present PMD mapping a folio again.
> +        */
> +       if (is_pmd_migration_entry(pmde))
> +               return SCAN_PMD_MAPPED;
>         if (!pmd_present(pmde))
>                 return SCAN_PMD_NULL;
>         if (pmd_trans_huge(pmde))
> --
> 2.30.2
>