mm/slub: fix memory leak in free_to_pcs_bulk()

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 <admin@simplepod.ai>
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>
Link: https://patch.msgid.link/20251111125331.12246-1-harry.yoo@oracle.com
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Tested-by: Darrick J. Wong <djwong@kernel.org>
Tested-by: Tytus Rogalewski <admin@simplepod.ai>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

diff --git a/mm/slub.c b/mm/slub.c
index f1a5373eee7b7d3fe76b457e2d21e4d66d8873b4..a787687a0d593f4c4f3dd13e6652a34b0944cc18 100644 (file)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -6332,8 +6332,6 @@ next_remote_batch:
 
                if (unlikely(!slab_free_hook(s, p[i], init, false))) {
                        p[i] = p[--size];
-                       if (!size)
-                               goto flush_remote;
                        continue;
                }
 
@@ -6348,6 +6346,9 @@ next_remote_batch:
                i++;
        }
 
+       if (!size)
+               goto flush_remote;
+
 next_batch:
        if (!local_trylock(&s->cpu_sheaves->lock))
                goto fallback;
@@ -6402,6 +6403,9 @@ do_free:
                goto next_batch;
        }
 
+       if (remote_nr)
+               goto flush_remote;
+
        return;
 
 no_empty: