[PATCH V1] mm/slub: fix memory leak in free_to_pcs_bulk()

[Harry Yoo <harry.yoo@oracle.com>]

The commit 989b09b73978 ("slab: skip percpu sheaves for remote object
freeing") introduced the remote_objects array in free_to_pcs_bulk() to
skip sheaves when objects from a remote node are freed.

However, the array is flushed only when:
  1) the array becomes full (++remote_nr >= PCS_BATCH_MAX), or
  2) slab_free_hook() returns false and size becomes zero.

When neither of the conditions is met, objects in the array are leaked.
This resulted in a memory leak [1], where 82 GiB of memory was allocated
for the maple_node cache.

Flush the array after successfully freeing objects to sheaves
in the do_free: path.

In the meantime, move the snippet if (!size) goto flush_remote; outside
the while loop for readability. Let's say all objects in the array are
from a remote node: then we acquire s->cpu_sheaves->lock and try to free
an object even when size is zero. This doesn't appear to be harmful,
but isn't really readable.

Reported-by: Tytus Rogalewski <tytanick@gmail.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220765 [1]
Closes: https://lore.kernel.org/linux-mm/20251107094809.12e9d705b7bf4815783eb184@linux-foundation.org
Closes: https://lore.kernel.org/all/aRGDTwbt2EIz2CYn@hyeyoo
Fixes: 989b09b73978 ("slab: skip percpu sheaves for remote object freeing")
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---
 mm/slub.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index f1a5373eee7b..a787687a0d59 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -6332,8 +6332,6 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
 
 		if (unlikely(!slab_free_hook(s, p[i], init, false))) {
 			p[i] = p[--size];
-			if (!size)
-				goto flush_remote;
 			continue;
 		}
 
@@ -6348,6 +6346,9 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
 		i++;
 	}
 
+	if (!size)
+		goto flush_remote;
+
 next_batch:
 	if (!local_trylock(&s->cpu_sheaves->lock))
 		goto fallback;
@@ -6402,6 +6403,9 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
 		goto next_batch;
 	}
 
+	if (remote_nr)
+		goto flush_remote;
+
 	return;
 
 no_empty:
-- 
2.43.0