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

[David Hildenbrand <david@redhat.com>]

On 03.07.25 07:48, 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@arm.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(+)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 1aa7ca67c756..3fdefc4f4984 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -941,6 +941,15 @@ 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 the PMD still pointing to a PMD-order folio, so return
> +	 * SCAN_PMD_MAPPED.

Nit: the last part (, so return ..) is obvious from the code.

I would have written

/*
  * 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 entry again.
  */


Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb