Re: [PATCH 1/2] mm: page_alloc: do not give all non-blocking requests reserve access

[Balbir Singh]

On Wed, Oct 28, 2026 at 01:15:53PM -0400, Johannes Weiner wrote:
> 281dd25c1a01 ("mm/page_alloc: let GFP_ATOMIC order-0 allocs access
> highatomic reserves") accidentally gave all non-blocking allocation
> requests access to highatomic reserves, which includes GFP_NOWAIT and
> other sites clearing __GFP_DIRECT_RECLAIM.
> 
> While this improved atomic request success rate, it's unintentionally
> broad and can actually worsen highatomic requests by squandering the
> reserves on requests that don't need it.
> 
> There are concurrent efforts to clear __GFP_DIRECT_RECLAIM altogether
> for costly order __GFP_NORETRY requests, which would then also fall
> into this exemption and deplete reserves even faster.
> 
> What GFP_ATOMIC has over the others is __GFP_HIGH, which
> alloc_flags_slowpath() translates to ALLOC_MIN_RESERVE.
> 
> Narrow the exemption to contexts that have both ALLOC_NON_BLOCK |
> ALLOC_MIN_RESERVE.
> 
> Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> Link: https://sashiko.dev/#/patchset/20260904115629.3993331-1-dipiets@xxxxxxxxx
> Fixes: 281dd25c1a01 ("mm/page_alloc: let GFP_ATOMIC order-0 allocs access highatomic reserves")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
>  mm/page_alloc.c | 4 +++-
>  mm/page_alloc.h | 3 +++
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index a8cac5e01066..8ac2e2bf14fc 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3238,7 +3238,9 @@ struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone,
>  			 * reserves as failing now is worse than failing a
>  			 * high-order atomic allocation in the future.
>  			 */
> -			if (!page && (alloc_flags & (ALLOC_OOM|ALLOC_NON_BLOCK)))
> +			if (!page &&
> +			    ((alloc_flags & ALLOC_OOM) ||
> +			     (alloc_flags & ALLOC_MASK_ATOMIC) == ALLOC_MASK_ATOMIC))
>  				page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);
>  
>  			if (!page) {
> diff --git a/mm/page_alloc.h b/mm/page_alloc.h
> index e238ee91ed24..9c3c2830acb0 100644
> --- a/mm/page_alloc.h
> +++ b/mm/page_alloc.h
> @@ -60,6 +60,9 @@
>  /* Flags that allow allocations below the min watermark. */
>  #define ALLOC_RESERVES (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM)
>  
> +/* Flags that mean GFP_ATOMIC */
> +#define ALLOC_MASK_ATOMIC (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE)
> +
>  /*
>   * Structure for holding the mostly immutable allocation parameters passed
>   * between functions involved in allocations, including the alloc_pages*

I feel patch 2 needs to come first, so that
__zone_watermark_unusable_free() accounts correctly.

Acked-by: Balbir Singh <balbirs@xxxxxxxxxx>