linux-kernel - Re: [RFC PATCH v1 1/7] mm: migrate: Allow misplaced migration without VMA too

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Message-ID: <87ectde5k3.fsf@DESKTOP-5N7EMDA>
Date: Fri, 15 Aug 2025 09:29:00 +0800
From: "Huang, Ying" <ying.huang@...ux.alibaba.com>
To: Bharata B Rao <bharata@....com>
Cc: <linux-kernel@...r.kernel.org>,  <linux-mm@...ck.org>,
  <Jonathan.Cameron@...wei.com>,  <dave.hansen@...el.com>,
  <gourry@...rry.net>,  <hannes@...xchg.org>,
  <mgorman@...hsingularity.net>,  <mingo@...hat.com>,
  <peterz@...radead.org>,  <raghavendra.kt@....com>,  <riel@...riel.com>,
  <rientjes@...gle.com>,  <sj@...nel.org>,  <weixugc@...gle.com>,
  <willy@...radead.org>,  <ziy@...dia.com>,  <dave@...olabs.net>,
  <nifan.cxl@...il.com>,  <xuezhengchu@...wei.com>,  <yiannis@...corp.com>,
  <akpm@...ux-foundation.org>,  <david@...hat.com>,  <byungchul@...com>,
  <kinseyho@...gle.com>,  <joshua.hahnjy@...il.com>,  <yuanchu@...gle.com>,
  <balbirs@...dia.com>
Subject: Re: [RFC PATCH v1 1/7] mm: migrate: Allow misplaced migration
 without VMA too

Bharata B Rao <bharata@....com> writes:

> We want isolation of misplaced folios to work in contexts
> where VMA isn't available. In order to prepare for that
> allow migrate_misplaced_folio_prepare() to be called with
> a NULL VMA.
>
> 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 425401b2d4e1..7e356c0b1b5a 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2619,7 +2619,8 @@ 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.

The locking rule is changed.  IMO, it deserves more explanation in patch description.

>   */
>  int migrate_misplaced_folio_prepare(struct folio *folio,
>  		struct vm_area_struct *vma, int node)
> @@ -2636,7 +2637,7 @@ int migrate_misplaced_folio_prepare(struct folio *folio,
>  		 * 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 && (vma->vm_flags & VM_EXEC) && folio_maybe_mapped_shared(folio))
>  			return -EACCES;
>  
>  		/*

---
Best Regards,
Huang, Ying