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

On Fri, Oct 31, 2025 at 10:34:33AM +0100, Christoph Hellwig wrote:

 /**
- * 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.

elem => elems (and likewise for mempool_free_bulk())

  * 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)

What exactly is the behavior on partial failures?  Is the return value 0
or is it -ENOMEM, and is the array restored to its original state or
might some elements have been allocated?

+/**
+ * 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);

How much overhead does this add to mempool_alloc(), which will continue
to be the common case?  I wonder if it would be worthwhile to
force-inline the bulk allocation function into it, so that it will get
generate about the same code as before.

The above comment has a weird position now.  Maybe move it into the
'else if' block below.

-	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);

- Eric