Re: [PATCH v3] mm/slub: free prefilled empty sheaves consistently

[Hao Li]

On Mon, May 25, 2026 at 09:10:00PM +0800, hu.shengming@xxxxxxxxxx wrote:
> From: Shengming Hu <hu.shengming@xxxxxxxxxx>
> 
> Empty sheaves allocated by alloc_empty_sheaf() should be released with
> free_empty_sheaf().
> 
> Move the empty-sheaf capacity initialization into __alloc_empty_sheaf()
> and use it for oversized prefilled sheaves as well.
> 
> This keeps the prefill paths consistent: after flushing an oversized or
> pfmemalloc sheaf, release the now-empty sheaf through free_empty_sheaf()
> instead of kfree().
> 
> Signed-off-by: Shengming Hu <hu.shengming@xxxxxxxxxx>
> ---
> Changes in v2:
> - Rework the change as suggested by Harry.
> - Teach __alloc_empty_sheaf() to initialize capacity and pfmemalloc.
> - Allocate oversized prefilled sheaves through __alloc_empty_sheaf().
> - Free flushed oversized and pfmemalloc sheaves through free_empty_sheaf().
> - Link to v1: https://lore.kernel.org/all/20260521195015105Y4zvKHj0TfPZEujixy9Vo@xxxxxxxxxx/
> 
> Changes in v3:
> - Address Hao's comments:
> - Drop the redundant `pfmemalloc` initialization in
>   __alloc_empty_sheaf().
> - Keep initializing `capacity` and `pfmemalloc` in the normal-sized
>   prefill path, since the sheaf may be reused rather than freshly
>   allocated.
> - Link to v2: https://lore.kernel.org/all/20260522145900248m-nBcy07_SCDk2ATDWfmg@xxxxxxxxxx/
> 
> ---
>  mm/slub.c | 13 ++++---------
>  1 file changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 04692a6f9128..f4c06bf43f2d 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2782,6 +2782,7 @@ static struct slab_sheaf *__alloc_empty_sheaf(struct kmem_cache *s, gfp_t gfp,
>  		return NULL;
> 
>  	sheaf->cache = s;
> +	sheaf->capacity = capacity;
> 
>  	stat(s, SHEAF_ALLOC);
> 
> @@ -5015,21 +5016,15 @@ kmem_cache_prefill_sheaf(struct kmem_cache *s, gfp_t gfp, unsigned int size)
> 
>  	if (unlikely(size > s->sheaf_capacity)) {
> 
> -		sheaf = kzalloc_flex(*sheaf, objects, size, gfp);
> +		sheaf = __alloc_empty_sheaf(s, gfp, size);
>  		if (!sheaf)
>  			return NULL;
> 
>  		stat(s, SHEAF_PREFILL_OVERSIZE);
> -		sheaf->cache = s;
> -		sheaf->capacity = size;
> 
> -		/*
> -		 * we do not need to care about pfmemalloc here because oversize
> -		 * sheaves area always flushed and freed when returned
> -		 */

just a bit curious, why did you decide to delete these comments?

>  		if (!__kmem_cache_alloc_bulk(s, gfp, size,
>  					     &sheaf->objects[0])) {
> -			kfree(sheaf);
> +			free_empty_sheaf(s, sheaf);
>  			return NULL;
>  		}
> 
> @@ -5097,7 +5092,7 @@ void kmem_cache_return_sheaf(struct kmem_cache *s, gfp_t gfp,
>  	if (unlikely((sheaf->capacity != s->sheaf_capacity)
>  		     || sheaf->pfmemalloc)) {
>  		sheaf_flush_unused(s, sheaf);
> -		kfree(sheaf);
> +		free_empty_sheaf(s, sheaf);
>  		return;
>  	}
> 
> -- 
> 2.25.1