Re: [PATCH v2] mm/vmscan: fix unintended mtc->nmask mutation in alloc_demote_folio()

[Lorenzo Stoakes]

On Tue, Mar 03, 2026 at 05:25:17AM +0000, Bing Jiao wrote:
> In alloc_demote_folio(), mtc->nmask is set to NULL for the first
> allocation. If that succeeds, it returns without restoring mtc->nmask
> to allowed_mask. For subsequent allocations from the migrate_pages()
> batch, mtc->nmask will be NULL. If the target node then becomes full,
> the fallback allocation will use nmask = NULL, allocating from any
> node allowed by the task cpuset, which for kswapd is all nodes.
>
> To address this issue, use a local copy of the mtc structure with
> nmask = NULL for the first allocation attempt specifically, ensuring
> the original mtc remains unmodified.
>
> Fixes: 320080272892 ("mm/demotion: demote pages according to allocation fallback order")
> Signed-off-by: Bing Jiao <bingjiao@xxxxxxxxxx>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

> ---
>  mm/vmscan.c | 14 +++++---------
>  1 file changed, 5 insertions(+), 9 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index cbffc0a27824..c4e0ce737e03 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -966,13 +966,11 @@ static void folio_check_dirty_writeback(struct folio *folio,
>  static struct folio *alloc_demote_folio(struct folio *src,
>  		unsigned long private)
>  {
> +	struct migration_target_control *mtc, target_nid_mtc;
>  	struct folio *dst;
> -	nodemask_t *allowed_mask;
> -	struct migration_target_control *mtc;
>
>  	mtc = (struct migration_target_control *)private;
>
> -	allowed_mask = mtc->nmask;
>  	/*
>  	 * make sure we allocate from the target node first also trying to
>  	 * demote or reclaim pages from the target node via kswapd if we are
> @@ -982,15 +980,13 @@ static struct folio *alloc_demote_folio(struct folio *src,
>  	 * a demotion of cold pages from the target memtier. This can result
>  	 * in the kernel placing hot pages in slower(lower) memory tiers.
>  	 */
> -	mtc->nmask = NULL;
> -	mtc->gfp_mask |= __GFP_THISNODE;
> -	dst = alloc_migration_target(src, (unsigned long)mtc);
> +	target_nid_mtc = *mtc;
> +	target_nid_mtc.nmask = NULL;
> +	target_nid_mtc.gfp_mask |= __GFP_THISNODE;
> +	dst = alloc_migration_target(src, (unsigned long)&target_nid_mtc);
>  	if (dst)
>  		return dst;
>
> -	mtc->gfp_mask &= ~__GFP_THISNODE;
> -	mtc->nmask = allowed_mask;
> -
>  	return alloc_migration_target(src, (unsigned long)mtc);
>  }
>
> --
> 2.53.0.473.g4a7958ca14-goog
>