Re: [PATCH v2] mm/slub: free prefilled empty sheaves consistently

[Hao Li]

On Fri, May 22, 2026 at 02:59:00PM +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 common empty-sheaf initialization into __alloc_empty_sheaf(),
> including the capacity and default pfmemalloc state, 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/
> 
> ---
>  mm/slub.c | 17 +++++------------
>  1 file changed, 5 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 04692a6f9128..bbe8351f1bfa 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2782,6 +2782,8 @@ static struct slab_sheaf *__alloc_empty_sheaf(struct kmem_cache *s, gfp_t gfp,
>  		return NULL;
> 
>  	sheaf->cache = s;
> +	sheaf->capacity = capacity;
> +	sheaf->pfmemalloc = false;

we could eliminate `sheaf->pfmemalloc = false;` because the allocation use
kzalloc.

> 
>  	stat(s, SHEAF_ALLOC);
> 
> @@ -5015,21 +5017,15 @@ 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
> -		 */
>  		if (!__kmem_cache_alloc_bulk(s, gfp, size,
>  					     &sheaf->objects[0])) {
> -			kfree(sheaf);
> +			free_empty_sheaf(s, sheaf);
>  			return NULL;
>  		}
> 
> @@ -5064,9 +5060,6 @@ kmem_cache_prefill_sheaf(struct kmem_cache *s, gfp_t gfp, unsigned int size)
>  		sheaf = alloc_empty_sheaf(s, gfp);
> 
>  	if (sheaf) {
> -		sheaf->capacity = s->sheaf_capacity;
> -		sheaf->pfmemalloc = false;

I think we couldn't delete 'sheaf->pfmemalloc = false;', because the 'sheaf'
pointer here could points to a normal size sheaf via the `if (pcs->spare) {}
else {}` statements above.

> -
>  		if (sheaf->size < size &&
>  		    __prefill_sheaf_pfmemalloc(s, sheaf, gfp)) {
>  			sheaf_flush_unused(s, sheaf);
> @@ -5097,7 +5090,7 @@ 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

-- 
Thanks,
Hao