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

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Message-ID: <82cb169b-15b1-46f9-9d8e-4c9483de1c77@nvidia.com>
Date: Mon, 24 Mar 2025 13:55:43 +1100
From: Balbir Singh <balbirs@...dia.com>
To: Bharata B Rao <bharata@....com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Cc: AneeshKumar.KizhakeVeetil@....com, Hasan.Maruf@....com,
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Subject: Re: [RFC PATCH 1/4] mm: migrate: Allow misplaced migration without
 VMA too

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@....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?

Balbir