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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/7/3 13:48, 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>

With David's comments addressed, LGTM.

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>