[PATCH v2] mm: mempool: fix wake-up edge case bug for zero-minimum pools

[Yadan Fan <ydfan@suse.com>]

The mempool wake-up path has a edge case bug that affects pools created
with min_nr=0. When a thread blocks waiting for memory from an empty
pool (curr_nr == 0), subsequent mempool_free() calls fail to wake
the waiting thread because the condition "curr_nr < min_nr" evaluates
to "0 < 0" which is false, this can cause threads to sleep indefinitely
according to the code logic.

There is at least 2 places where the mempool created with min_nr=0:

1. lib/btree.c:191: mempool_create(0, btree_alloc, btree_free, NULL)
2. drivers/md/dm-verity-fec.c:791:
 mempool_init_slab_pool(&f->extra_pool, 0, f->cache)

Add an explicit check in mempool_free() to handle the min_nr=0 case:
when the pool has zero minimum reserves, is currently empty, and has
active waiters, allocate the element then wake up the sleeper.

Changes in v2:
- Inline the same logic from previous test for zero min_nr pool to return
  an element to the pool
- Inline the necessary parts of add_element() to avoid the BUG_ON which
  refuses the scenario of curr_nr == min_nr
- Use wq_has_sleeper() instead of unconditional wake_up() calls for better
  performance when no waiters are present

Signed-off-by: Yadan Fan <ydfan@suse.com>
---
 mm/mempool.c | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/mm/mempool.c b/mm/mempool.c
index 3223337135d0..204a216b6418 100644
--- a/mm/mempool.c
+++ b/mm/mempool.c
@@ -540,11 +540,43 @@ void mempool_free(void *element, mempool_t *pool)
 		if (likely(pool->curr_nr < pool->min_nr)) {
 			add_element(pool, element);
 			spin_unlock_irqrestore(&pool->lock, flags);
-			wake_up(&pool->wait);
+			if (wq_has_sleeper(&pool->wait))
+				wake_up(&pool->wait);
 			return;
 		}
 		spin_unlock_irqrestore(&pool->lock, flags);
 	}
+
+	/*
+	 * Handle the min_nr = 0 edge case:
+	 *
+	 * For zero-minimum pools, curr_nr < min_nr (0 < 0) never succeeds,
+	 * so waiters sleeping on pool->wait would never be woken by the
+	 * wake-up path of previous test. This explicit check ensures the
+	 * allocation of element when both min_nr and curr_nr are 0, and
+	 * any active waiters are properly awakened.
+	 *
+	 * Inline the same logic as previous test, add_element() cannot be
+	 * directly used here since it has BUG_ON to deny if min_nr equals
+	 * curr_nr, so here picked rest of add_element() to use without
+	 * BUG_ON check.
+	 */
+	if (unlikely(pool->min_nr == 0 &&
+		     READ_ONCE(pool->curr_nr) == 0)) {
+		spin_lock_irqsave(&pool->lock, flags);
+		if (likely(pool->curr_nr == 0)) {
+			/* Inline the logic of add_element() */
+			poison_element(pool, element);
+			if (kasan_poison_element(pool, element))
+				pool->elements[pool->curr_nr++] = element;
+			spin_unlock_irqrestore(&pool->lock, flags);
+			if (wq_has_sleeper(&pool->wait))
+				wake_up(&pool->wait);
+			return;
+		}
+		spin_unlock_irqrestore(&pool->lock, flags);
+	}
+
 	pool->free(element, pool->pool_data);
 }
 EXPORT_SYMBOL(mempool_free);
-- 
2.50.1