Re: [PATCH v3] mm/slub: prevent pfmemalloc objects from entering the barn

[Harry Yoo]

On 7/21/26 9:45 AM, hu.shengming@xxxxxxxxxx wrote:
> From: Shengming Hu <hu.shengming@xxxxxxxxxx>
> 
> kmem_cache_return_sheaf() may refill a partially consumed sheaf before
> placing it in the barn. Without an explicit restriction, this refill may
> draw objects from pfmemalloc slabs and consume emergency reserves.
> 
> Add __GFP_NOMEMALLOC so that returned sheaves are refilled only from
> non-pfmemalloc slabs. Also add __GFP_NOWARN, as suggested by Hao Li,
> because this refill is a best-effort attempt and failure is acceptable.
> If the refill fails, flush and free the sheaf instead.
> 
> Fixes: 3c1ea5c5019f ("slab: sheaf prefilling for guaranteed allocations")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Shengming Hu <hu.shengming@xxxxxxxxxx>
> ---

Looks good to me,
Reviewed-by: Harry Yoo (Oracle) <harry@xxxxxxxxxx>

> Changes in v3:
> - Replace “normal memory” with the more precise “non-pfmemalloc slabs”,
>   and add Fixes: and Cc: stable@xxxxxxxxxxxxxxx, as suggested by Harry.
> - Link to v2: https://lore.kernel.org/all/20260720211127124sYtTNbvsxCXN9_pDh-pwt@xxxxxxxxxx/
> 
> Changes in v2:
> - Add __GFP_NOWARN as suggested by Hao.
> - Link to v1: https://lore.kernel.org/all/20260719113701797vZ3R8NxiqYt8dEfeUxtON@xxxxxxxxxx/
> 
> ---
>  mm/slub.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 53b4976d3831..357b7522c817 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5123,7 +5123,7 @@ void kmem_cache_return_sheaf(struct kmem_cache *s, gfp_t gfp,
>  	 * simply flush and free it.
>  	 */
>  	if (!barn || data_race(barn->nr_full) >= MAX_FULL_SHEAVES ||
> -	    refill_sheaf(s, sheaf, gfp)) {
> +	    refill_sheaf(s, sheaf, gfp | __GFP_NOMEMALLOC | __GFP_NOWARN)) {
>  		sheaf_flush_unused(s, sheaf);
>  		free_empty_sheaf(s, sheaf);
>  		return;

-- 
Cheers,
Harry / Hyeonggon

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