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

[Yadan Fan]

The mempool wake-up mechanism 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 causes threads to sleep indefinitely.

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, wake them up. The wq_has_sleeper() avoids unnecessary
wake-ups when no threads are waiting.

Signed-off-by: Yadan Fan <ydfan@xxxxxxxx>
---
 mm/mempool.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/mm/mempool.c b/mm/mempool.c
index 3223337135d0..803f8853e0f1 100644
--- a/mm/mempool.c
+++ b/mm/mempool.c
@@ -545,6 +545,22 @@ void mempool_free(void *element, mempool_t *pool)
                }
                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
+        * normal wake-up path. This explicit check ensures that when
+        * pool->min_nr == 0 and pool->curr_nr == 0, any active waiters
+        * are properly awakened.
+        * The wq_has_sleeper() avoids unnecessary wake-ups when no
+        * threads are waiting.
+        */
+       if (unlikely(pool->min_nr == 0 &&
+                    READ_ONCE(pool->curr_nr) == 0 &&
+                    wq_has_sleeper(&pool->wait))) {
+               wake_up(&pool->wait);
+       }
+
        pool->free(element, pool->pool_data);
 }
 EXPORT_SYMBOL(mempool_free);
--
2.50.1