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

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Sep 10, 2026 at 08:18:24PM -0400, Gregory Price wrote:
> From: "Gregory Price (Meta)" <gourry@gourry.net>
> 
> 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@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
> ---
>  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;
>  

Semi related; I've often wondered if we should still account shared and
pinned vmas in the fault statistic, even though we should not migrate
them.

After all, those pages are still used and by not accounting them in the
fault statistics, it becomes easier to migrate a task away from them.

Using the scanning for two different things has made a mess of things
though :/