To migrate unmapped pagecache folios, migrate_misplaced_folio and
migrate_misplaced_folio_prepare must handle folios without VMAs.

migrate_misplaced_folio_prepare checks VMA for exec bits, so allow
a NULL VMA when it does not have a mapping.

migrate_misplaced_folio must call migrate_pages with MIGRATE_SYNC
when in the pagecache path because it is a synchronous context.

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Gregory Price <gourry@gourry.net>
---
 mm/migrate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index dfb5eba3c522..3b0bd3f21ac3 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2632,7 +2632,7 @@ 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;
 
-- 
2.43.0

On 11/27/2024 1:51 PM, Gregory Price wrote:
> To migrate unmapped pagecache folios, migrate_misplaced_folio and
> migrate_misplaced_folio_prepare must handle folios without VMAs.
> 
> migrate_misplaced_folio_prepare checks VMA for exec bits, so allow
> a NULL VMA when it does not have a mapping.
> 
> migrate_misplaced_folio must call migrate_pages with MIGRATE_SYNC
> when in the pagecache path because it is a synchronous context.
> 
> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Gregory Price <gourry@gourry.net>
> ---
>   mm/migrate.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index dfb5eba3c522..3b0bd3f21ac3 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2632,7 +2632,7 @@ 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;
>   

Thanks for this patch.

This would be helpful in the cases of independent page scanning
algorithms where we do not have a VMA associated with that.

Hopefully it can be taken to tree independently.
Feel free to add.

Reviewed-by: Raghavendra K T <raghavendra.kt@amd.com>

Hi, Gregory,

Gregory Price <gourry@gourry.net> writes:

> To migrate unmapped pagecache folios, migrate_misplaced_folio and
> migrate_misplaced_folio_prepare must handle folios without VMAs.

IMHO, it's better to use migrate_misplaced_folio() instead of
migrate_misplaced_folio for readability in patch title and description.

> migrate_misplaced_folio_prepare checks VMA for exec bits, so allow
> a NULL VMA when it does not have a mapping.
>
> migrate_misplaced_folio must call migrate_pages with MIGRATE_SYNC
> when in the pagecache path because it is a synchronous context.

I don't find the corresponding implementation for this.  And, I don't
think it's a good idea to change from MIGRATE_ASYNC to MIGRATE_SYNC.
This may cause too long page access latency for page placement
optimization.  The downside may offset the benefit.

And, it appears that we can delete the "vma" parameter of
migrate_misplaced_folio() because it's not used now.  This is a trivial
code cleanup.

> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Gregory Price <gourry@gourry.net>
> ---
>  mm/migrate.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index dfb5eba3c522..3b0bd3f21ac3 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2632,7 +2632,7 @@ 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;

---
Best Regards,
Huang, Ying

On Thu, Nov 28, 2024 at 07:12:11PM +0800, Huang, Ying wrote:
> Hi, Gregory,
> 
> Gregory Price <gourry@gourry.net> writes:
> 
> > To migrate unmapped pagecache folios, migrate_misplaced_folio and
> > migrate_misplaced_folio_prepare must handle folios without VMAs.
> 
> IMHO, it's better to use migrate_misplaced_folio() instead of
> migrate_misplaced_folio for readability in patch title and description.
> 
> > migrate_misplaced_folio_prepare checks VMA for exec bits, so allow
> > a NULL VMA when it does not have a mapping.
> >
> > migrate_misplaced_folio must call migrate_pages with MIGRATE_SYNC
> > when in the pagecache path because it is a synchronous context.
> 
> I don't find the corresponding implementation for this.  And, I don't
> think it's a good idea to change from MIGRATE_ASYNC to MIGRATE_SYNC.
> This may cause too long page access latency for page placement
> optimization.  The downside may offset the benefit.
> 
> And, it appears that we can delete the "vma" parameter of
> migrate_misplaced_folio() because it's not used now.  This is a trivial
> code cleanup.
>

This patch apparently got a bit away from me and was heavily reduced
from its initial form.  This commit message is just wrong now.  I will
update this and the 2nd commit and probably submit them separately.

~Gregory