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

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Message-ID: <2ad7973b-9d92-4b34-8534-73ed1164a1c8@redhat.com>
Date: Thu, 6 Mar 2025 13:13:23 +0100
From: David Hildenbrand <david@...hat.com>
To: Bharata B Rao <bharata@....com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Cc: AneeshKumar.KizhakeVeetil@....com, Hasan.Maruf@....com,
 Jonathan.Cameron@...wei.com, Michael.Day@....com, akpm@...ux-foundation.org,
 dave.hansen@...el.com, feng.tang@...el.com, gourry@...rry.net,
 hannes@...xchg.org, honggyu.kim@...com, hughd@...gle.com,
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Subject: Re: [RFC PATCH 1/4] mm: migrate: Allow misplaced migration without
 VMA too

On 06.03.25 06:45, 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.

I was initially confused about "can be called", because it can't

Consider phrasing it as "We want to make use of 
alloc_misplaced_dst_folio() in context where we don't have VMA 
information available. To prepare for that ..."

> 
> 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;
>   


-- 
Cheers,

David / dhildenb