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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 9/17/26 19:49, Zi Yan wrote:
> On Thu Sep 10, 2026 at 8:18 PM EDT, 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;
>>  
>>  		/*
> 
> Should we rename folio_use_access_time() to folio_in_lowtier()?
> Otherwise the code is really hard to understand. 

I just stumbled over that myself and I agree.

> 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.

We can just have an alias function if it makes the code easier to get.

-- 
Cheers,

David