Re: [RFC PATCH 1/4] mm: migrate: Allow misplaced migration without VMA too

Bharata B Rao
2025-03-24T14:51:40Z

On 24-Mar-25 8:25 AM, Balbir Singh wrote:
> On 3/6/25 16:45, Bharata B Rao wrote:
>> migrate_misplaced_folio_prepare() can be called from a
>> context where VMA isn't available. Allow the migration
>> to work from such contexts too.
>>
>> Signed-off-by: Bharata B Rao <bharata@amd.com>
>> ---
>> mm/migrate.c | 5 +++--
>> 1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index fb19a18892c8..5b21856a0dd0 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -2639,7 +2639,8 @@ static struct folio *alloc_misplaced_dst_folio(struct folio *src,
>>
>> /*
>> * Prepare for calling migrate_misplaced_folio() by isolating the folio if
>> - * permitted. Must be called with the PTL still held.
>> + * permitted. Must be called with the PTL still held if called with a non-NULL
>> + * vma.
>> */
>> int migrate_misplaced_folio_prepare(struct folio *folio,
>> struct vm_area_struct *vma, int node)
>> @@ -2656,7 +2657,7 @@ int migrate_misplaced_folio_prepare(struct folio *folio,
>> * See folio_likely_mapped_shared() on possible imprecision
>> * when we cannot easily detect if a folio is shared.
>> */
>> - if ((vma->vm_flags & VM_EXEC) &&
>> + if (vma && (vma->vm_flags & VM_EXEC) &&
>> folio_likely_mapped_shared(folio))
>> return -EACCES;
>>
>
> In the worst case, the absence of the vma would mean that we try to isolate
> and migrate a shared folio with executable pages. Are those a key target for the
> hot page migration?

I don't think they are a key target for hot page migration, but if
shared executable pages (like shared library pages) have ended up in
lower tier, it doesn't hurt to get them promoted to top tier, I would think.

Regards,
Bharata.