mempool: legitimize the io_schedule_timeout in mempool_alloc_from_pool

The timeout here is and old workaround with a Fixme comment.  But
thinking about it, it makes sense to keep it, so reword the comment.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20251113084022.1255121-9-hch@lst.de
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Author: Christoph Hellwig

mm/mempool.c

@@ -427,10 +427,10 @@ static unsigned int mempool_alloc_from_pool(struct mempool *pool, void **elems,
 		spin_unlock_irqrestore(&pool->lock, flags);
 
 		/*
-		 * Wait for someone else to return an element to @pool.
-		 *
-		 * FIXME: this should be io_schedule().  The timeout is there as
-		 * a workaround for some DM problems in 2.6.18.
+		 * Wait for someone else to return an element to @pool, but wake
+		 * up occasionally as memory pressure might have reduced even
+		 * and the normal allocation in alloc_fn could succeed even if
+		 * no element was returned.
 		 */
 		io_schedule_timeout(5 * HZ);
 		finish_wait(&pool->wait, &wait);