Re: [PATCH v2 2/4] mm: allow shared folios to be promoted to a fast tier

[Zi Yan]

On Thu Sep 10, 2026 at 8:18 PM EDT, Gregory Price wrote:
> From: "Gregory Price (Meta)" <gourry@xxxxxxxxxx>
>
> NUMA balancing rejects shared copy-on-write folios and executable
> file folios mapped by multiple processes to avoid placement bouncing.
> These checks also block promotion from slow memory.
>
> Allow such folios to participate when moving from a slow tier to a fast
> tier. Keep the existing restrictions for ordinary placement.
>
> Fixes: c574bbe91703 ("NUMA balancing: optimize page placement for memory tiering system")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: LLM
> Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
> ---
>  mm/mempolicy.c | 8 ++++++--
>  mm/migrate.c   | 6 ++++--
>  2 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index a082ccfa09ec..19b599bc2dd1 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -863,8 +863,12 @@ bool folio_can_map_prot_numa(struct folio *folio, struct vm_area_struct *vma,
>  	if (!folio || folio_is_zone_device(folio) || folio_test_ksm(folio))
>  		return false;
>  
> -	/* Also skip shared copy-on-write folios */
> -	if (vma_is_cow_mapping(vma) && folio_maybe_mapped_shared(folio))
> +	/*
> +	 * Shared copy-on-write folios are poor NUMA placement candidates, but
> +	 * a hot folio on a slow tier still needs a hint fault for promotion.
> +	 */
> +	if (vma_is_cow_mapping(vma) && folio_maybe_mapped_shared(folio) &&
> +	    !folio_use_access_time(folio))
>  		return false;
>  
>  	/* Folios are pinned and can't be migrated */
> diff --git a/mm/migrate.c b/mm/migrate.c
> index a369d0c95c38..afd9c97d2389 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2697,12 +2697,14 @@ int migrate_misplaced_folio_prepare(struct folio *folio,
>  		/*
>  		 * Do not migrate file folios that are mapped in multiple
>  		 * processes with execute permissions as they are probably
> -		 * shared libraries.
> +		 * shared libraries, unless this is a promotion from a slow tier.
>  		 *
>  		 * See folio_maybe_mapped_shared() on possible imprecision
>  		 * when we cannot easily detect if a folio is shared.
>  		 */
> -		if ((vma->vm_flags & VM_EXEC) && folio_maybe_mapped_shared(folio))
> +		if ((vma->vm_flags & VM_EXEC) &&
> +		    folio_maybe_mapped_shared(folio) &&
> +		    (!folio_use_access_time(folio) || !node_is_toptier(node)))
>  			return -EACCES;
>  
>  		/*

Should we rename folio_use_access_time() to folio_in_lowtier()?
Otherwise the code is really hard to understand. I admit that I
introduced folio_use_access_time() and it was probably because it
decides the use of folio_xchg_access_time() in
folio_can_map_prot_numa(). But in the other callsites, folio_in_lowtier()
makes more sense.

After the rename, a comment "only record access time of folios in low tier"
above folio_xchg_access_time() should work.

-- 
Best Regards,
Yan, Zi