Re: [PATCH 3/9] mempool: add mempool_{alloc,free}_bulk

Vlastimil Babka
2025-11-05T15:04:56Z

On 10/31/25 10:34, Christoph Hellwig wrote:
> Add a version of the mempool allocator that works for batch allocations
> of multiple objects. Calling mempool_alloc in a loop is not safe because
> it could deadlock if multiple threads are performing such an allocation
> at the same time.
>
> As an extra benefit the interface is build so that the same array can be
> used for alloc_pages_bulk / release_pages so that at least for page
> backed mempools the fast path can use a nice batch optimization.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> include/linux/mempool.h | 7 ++
> mm/mempool.c | 145 ++++++++++++++++++++++++++++------------
> 2 files changed, 111 insertions(+), 41 deletions(-)
>
> diff --git a/include/linux/mempool.h b/include/linux/mempool.h
> index 34941a4b9026..486ed50776db 100644
> --- a/include/linux/mempool.h
> +++ b/include/linux/mempool.h
> @@ -66,9 +66,16 @@ extern void mempool_destroy(mempool_t *pool);
> extern void *mempool_alloc_noprof(mempool_t *pool, gfp_t gfp_mask) __malloc;
> #define mempool_alloc(...) \
> alloc_hooks(mempool_alloc_noprof(__VA_ARGS__))
> +int mempool_alloc_bulk_noprof(mempool_t *pool, void **elem,
> + unsigned int count, gfp_t gfp_mask, unsigned long caller_ip);
> +#define mempool_alloc_bulk(pool, elem, count, gfp_mask) \
> + alloc_hooks(mempool_alloc_bulk_noprof(pool, elem, count, gfp_mask, \
> + _RET_IP_))
>
> extern void *mempool_alloc_preallocated(mempool_t *pool) __malloc;
> extern void mempool_free(void *element, mempool_t *pool);
> +unsigned int mempool_free_bulk(mempool_t *pool, void **elem,
> + unsigned int count);
>
> /*
> * A mempool_alloc_t and mempool_free_t that get the memory from
> diff --git a/mm/mempool.c b/mm/mempool.c
> index 15581179c8b9..c980a0396986 100644
> --- a/mm/mempool.c
> +++ b/mm/mempool.c
> @@ -381,23 +381,29 @@ int mempool_resize(mempool_t *pool, int new_min_nr)
> EXPORT_SYMBOL(mempool_resize);
>
> /**
> - * mempool_alloc - allocate an element from a memory pool
> + * mempool_alloc_bulk - allocate multiple elements from a memory pool
> * @pool: pointer to the memory pool
> + * @elem: partially or fully populated elements array
> + * @count: size (in entries) of @elem
> * @gfp_mask: GFP_* flags.
> *
> + * Allocate elements for each slot in @elem that is non-%NULL.
> + *
> * Note: This function only sleeps if the alloc_fn callback sleeps or returns
> * %NULL. Using __GFP_ZERO is not supported.
> *
> - * Return: pointer to the allocated element or %NULL on error. This function
> - * never returns %NULL when @gfp_mask allows sleeping.
> + * Return: 0 if successful, else -ENOMEM. This function never returns -ENOMEM
> + * when @gfp_mask allows sleeping.
> */
> -void *mempool_alloc_noprof(mempool_t *pool, gfp_t gfp_mask)
> +int mempool_alloc_bulk_noprof(struct mempool *pool, void **elem,
> + unsigned int count, gfp_t gfp_mask, unsigned long caller_ip)
> {
> - void *element;
> unsigned long flags;
> wait_queue_entry_t wait;
> gfp_t gfp_temp;
> + unsigned int i;
>
> + VM_WARN_ON_ONCE(count > pool->min_nr);
> VM_WARN_ON_ONCE(gfp_mask & __GFP_ZERO);
> might_alloc(gfp_mask);
>
> @@ -407,20 +413,31 @@ void *mempool_alloc_noprof(mempool_t *pool, gfp_t gfp_mask)
>
> gfp_temp = gfp_mask & ~(__GFP_DIRECT_RECLAIM|__GFP_IO);
>
> + i = 0;
> repeat_alloc:
> - if (should_fail_ex(&fail_mempool_alloc, 1, FAULT_NOWARN)) {
> - pr_info("forcing mempool usage for pool %pS\n",
> - (void *)_RET_IP_);
> - element = NULL;
> - } else {
> - element = pool->alloc(gfp_temp, pool->pool_data);
> - if (likely(element != NULL))
> - return element;
> + for (; i < count; i++) {
> + if (!elem[i]) {
> + if (should_fail_ex(&fail_mempool_alloc, 1,
> + FAULT_NOWARN)) {
> + pr_info("forcing pool usage for pool %pS\n",
> + (void *)caller_ip);
> + goto use_pool;
> + }

Would it be enough to do this failure injection attempt once and not in
every iteration?

> + elem[i] = pool->alloc(gfp_temp, pool->pool_data);
> + if (unlikely(!elem[i]))
> + goto use_pool;
> + }
> }
>
> + return 0;
> +
> +use_pool:
> spin_lock_irqsave(&pool->lock, flags);
> - if (likely(pool->curr_nr)) {
> - element = remove_element(pool);
> + if (likely(pool->curr_nr >= count - i)) {
> + for (; i < count; i++) {
> + if (!elem[i])
> + elem[i] = remove_element(pool);
> + }
> spin_unlock_irqrestore(&pool->lock, flags);
> /* paired with rmb in mempool_free(), read comment there */
> smp_wmb();
> @@ -428,8 +445,9 @@ void *mempool_alloc_noprof(mempool_t *pool, gfp_t gfp_mask)
> * Update the allocation stack trace as this is more useful
> * for debugging.
> */
> - kmemleak_update_trace(element);
> - return element;
> + for (i = 0; i < count; i++)
> + kmemleak_update_trace(elem[i]);
> + return 0;
> }
>
> /*
> @@ -445,10 +463,12 @@ void *mempool_alloc_noprof(mempool_t *pool, gfp_t gfp_mask)
> /* We must not sleep if !__GFP_DIRECT_RECLAIM */
> if (!(gfp_mask & __GFP_DIRECT_RECLAIM)) {
> spin_unlock_irqrestore(&pool->lock, flags);
> - return NULL;
> + if (i > 0)
> + mempool_free_bulk(pool, elem + i, count - i);

I don't understand why we are trying to free from i to count and not from 0
to i? Seems buggy, there will likely be NULLs which might go through
add_element() which assumes they are not NULL.

Assuming this is fixed we might still have confusing API. We might be
freeing away elements that were already in the array when
mempool_alloc_bulk() was called. OTOH the pool might be missing less than i
elements and mempool_free_bulk() will not do anything with the rest.
Anything beyond i is untouched. The caller has no idea what's in the array
after getting this -ENOMEM. (alloc_pages_bulk() returns the number of pages
there).
Maybe it's acceptable (your usecase I think doesn't even add a caller that
can't block), but needs documenting clearly.

> + return -ENOMEM;
> }
>
> - /* Let's wait for someone else to return an element to @pool */
> + /* Let's wait for someone else to return elements to @pool */
> init_wait(&wait);
> prepare_to_wait(&pool->wait, &wait, TASK_UNINTERRUPTIBLE);

So in theory callers waiting for many objects might wait indefinitely to
find enough objects in the pool, while smaller callers succeed their
allocations and deplete the pool. Mempools never provided some fair ordering
of waiters, but this might make it worse deterministically instead of
randomly. Guess it's not such a problem if all callers are comparable in
number of objects.

> @@ -463,6 +483,27 @@ void *mempool_alloc_noprof(mempool_t *pool, gfp_t gfp_mask)
> finish_wait(&pool->wait, &wait);
> goto repeat_alloc;
> }
> +EXPORT_SYMBOL_GPL(mempool_alloc_bulk_noprof);
> +
> +/**
> + * mempool_alloc - allocate an element from a memory pool
> + * @pool: pointer to the memory pool
> + * @gfp_mask: GFP_* flags.
> + *
> + * Note: This function only sleeps if the alloc_fn callback sleeps or returns
> + * %NULL. Using __GFP_ZERO is not supported.
> + *
> + * Return: pointer to the allocated element or %NULL on error. This function
> + * never returns %NULL when @gfp_mask allows sleeping.
> + */
> +void *mempool_alloc_noprof(struct mempool *pool, gfp_t gfp_mask)
> +{
> + void *elem[1] = { };
> +
> + if (mempool_alloc_bulk_noprof(pool, elem, 1, gfp_mask, _RET_IP_) < 0)
> + return NULL;
> + return elem[0];
> +}
> EXPORT_SYMBOL(mempool_alloc_noprof);
>
> /**
> @@ -502,21 +543,26 @@ void *mempool_alloc_preallocated(mempool_t *pool)
> EXPORT_SYMBOL(mempool_alloc_preallocated);
>
> /**
> - * mempool_free - return an element to a mempool
> - * @element: pointer to element
> + * mempool_free_bulk - return elements to a mempool
> * @pool: pointer to the memory pool
> + * @elem: elements to return
> + * @count: number of elements to return
> *
> - * Returns @elem to @pool if its needs replenishing, else free it using
> - * the free_fn callback in @pool.
> + * Returns elements from @elem to @pool if its needs replenishing and sets
> + * their slot in @elem to NULL. Other elements are left in @elem.
> + *
> + * Return: number of elements transferred to @pool. Elements are always
> + * transferred from the beginning of @elem, so the return value can be used as
> + * an offset into @elem for the freeing the remaining elements in the caller.
> *
> * This function only sleeps if the free_fn callback sleeps.

This part now only applies to mempool_free() ?

> */
> -void mempool_free(void *element, mempool_t *pool)
> +unsigned int mempool_free_bulk(struct mempool *pool, void **elem,
> + unsigned int count)
> {
> unsigned long flags;
> -
> - if (unlikely(element == NULL))
> - return;
> + bool added = false;
> + unsigned int freed = 0;
>
> /*
> * Paired with the wmb in mempool_alloc(). The preceding read is
> @@ -553,15 +599,11 @@ void mempool_free(void *element, mempool_t *pool)
> */
> if (unlikely(READ_ONCE(pool->curr_nr) < pool->min_nr)) {
> spin_lock_irqsave(&pool->lock, flags);
> - if (likely(pool->curr_nr < pool->min_nr)) {
> - add_element(pool, element);
> - spin_unlock_irqrestore(&pool->lock, flags);
> - if (wq_has_sleeper(&pool->wait))
> - wake_up(&pool->wait);
> - return;
> + while (pool->curr_nr < pool->min_nr && freed < count) {
> + add_element(pool, elem[freed++]);
> + added = true;
> }
> spin_unlock_irqrestore(&pool->lock, flags);
> - }
>
> /*
> * Handle the min_nr = 0 edge case:
> @@ -572,20 +614,41 @@ void mempool_free(void *element, mempool_t *pool)
> * allocation of element when both min_nr and curr_nr are 0, and
> * any active waiters are properly awakened.
> */
> - if (unlikely(pool->min_nr == 0 &&
> + } else if (unlikely(pool->min_nr == 0 &&
> READ_ONCE(pool->curr_nr) == 0)) {
> spin_lock_irqsave(&pool->lock, flags);
> if (likely(pool->curr_nr == 0)) {
> - add_element(pool, element);
> - spin_unlock_irqrestore(&pool->lock, flags);
> - if (wq_has_sleeper(&pool->wait))
> - wake_up(&pool->wait);
> - return;
> + add_element(pool, elem[freed++]);
> + added = true;
> }
> spin_unlock_irqrestore(&pool->lock, flags);
> }
>
> - pool->free(element, pool->pool_data);
> + if (unlikely(added) && wq_has_sleeper(&pool->wait))
> + wake_up(&pool->wait);
> +
> + return freed;
> +}
> +EXPORT_SYMBOL_GPL(mempool_free_bulk);
> +
> +/**
> + * mempool_free - return an element to the pool.
> + * @element: element to return
> + * @pool: pointer to the memory pool
> + *
> + * Returns @elem to @pool if its needs replenishing, else free it using
> + * the free_fn callback in @pool.
> + *
> + * This function only sleeps if the free_fn callback sleeps.
> + */
> +void mempool_free(void *element, struct mempool *pool)
> +{
> + if (likely(element)) {
> + void *elem[1] = { element };
> +
> + if (!mempool_free_bulk(pool, elem, 1))
> + pool->free(element, pool->pool_data);
> + }
> }
> EXPORT_SYMBOL(mempool_free);
>