Re: [PATCH] mm/mempool.c: set @elems slots to NULL once freed

[Vlastimil Babka <vbabka@suse.cz>]

On 12/24/25 00:10, klourencodev@gmail.com wrote:
> From: Kevin Lourenco <klourencodev@gmail.com>
> 
> This is documented in the function comment as "...and sets their
> slots in @elems to NULL.", but it was not followed.
> 
> We need to follow the NULL assignment, because elements newly returned
> to the pool must not be touched under any circumstances by the user.

Well alternatively we could update the comment if no user needs to rely on
this. Since it was modeled after __alloc_pages_bulk()/release_pages() and
release_pages() doesn't do this, we could also avoid it for perf reasons.
Christoph, what do you think?

> Signed-off-by: Kevin Lourenco <klourencodev@gmail.com>
> ---
>  mm/mempool.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mempool.c b/mm/mempool.c
> index c290e5261b47..1a2060304fd4 100644
> --- a/mm/mempool.c
> +++ b/mm/mempool.c
> @@ -674,7 +674,8 @@ unsigned int mempool_free_bulk(struct mempool *pool, void **elems,
>  	if (unlikely(READ_ONCE(pool->curr_nr) < pool->min_nr)) {
>  		spin_lock_irqsave(&pool->lock, flags);
>  		while (pool->curr_nr < pool->min_nr && freed < count) {
> -			add_element(pool, elems[freed++]);
> +			add_element(pool, elems[freed]);
> +			elems[freed++] = NULL;
>  			added = true;
>  		}
>  		spin_unlock_irqrestore(&pool->lock, flags);
> @@ -683,7 +684,8 @@ unsigned int mempool_free_bulk(struct mempool *pool, void **elems,
>  		/* Handle the min_nr = 0 edge case: */
>  		spin_lock_irqsave(&pool->lock, flags);
>  		if (likely(pool->curr_nr == 0)) {
> -			add_element(pool, elems[freed++]);
> +			add_element(pool, elems[freed]);
> +			elems[freed++] = NULL;
>  			added = true;
>  		}
>  		spin_unlock_irqrestore(&pool->lock, flags);