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

Darrick J. Wong
2025-11-12T18:46:49Z

On Tue, Nov 11, 2025 at 09:53:31PM +0900, Harry Yoo wrote:
> 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.

I'll put this on my test fleet this evening. Thank you for the quick
fix! :)

--D

> 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
>