Re: [PATCH] slab: Fix obj_ext is mistakenly considered NULL due to race condition

Harry Yoo
2025-10-23T02:25:10Z

On Thu, Oct 23, 2025 at 09:21:17AM +0800, Hao Ge wrote:
> From: Hao Ge <gehao@kylinos.cn>
>
> If two competing threads enter alloc_slab_obj_exts(), and the
> thread that failed to allocate the object extension vector exits
> after the one that succeeded, it will mistakenly assume slab->obj_ext
> is still empty due to its own allocation failure. This will then trigger
> warnings enforced by CONFIG_MEM_ALLOC_PROFILING_DEBUG checks in
> the subsequent free path.
>
> Therefore, let's add an additional check when alloc_slab_obj_exts fails.
>
> Signed-off-by: Hao Ge <gehao@kylinos.cn>
> ---
> mm/slub.c | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index d4403341c9df..42276f0cc920 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2227,9 +2227,12 @@ prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p)
> slab = virt_to_slab(p);
> if (!slab_obj_exts(slab) &&
> alloc_slab_obj_exts(slab, s, flags, false)) {
> - pr_warn_once("%s, %s: Failed to create slab extension vector!\n",
> - __func__, s->name);
> - return NULL;
> + /* Recheck if a racing thread has successfully allocated slab->obj_exts. */
> + if (!slab_obj_exts(slab)) {
> + pr_warn_once("%s, %s: Failed to create slab extension vector!\n",
> + __func__, s->name);
> + return NULL;
> + }
> }

Maybe this patch is a bit paranoid... since if mark_failed_objexts_alloc()
win cmpxchg() and then someone else allocates the object extension vector,
the warning will still be printed anyway.

But anyway, I think there is a better way to do this:

diff --git a/mm/slub.c b/mm/slub.c
index dd4c85ea1038..d08d7580349d 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2052,9 +2052,9 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
}
}

-static inline void mark_failed_objexts_alloc(struct slab *slab)
+static inline bool mark_failed_objexts_alloc(struct slab *slab)
{
- cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL);
+ return cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL) == 0;
}

static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
@@ -2076,7 +2076,7 @@ static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
#else /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */

static inline void mark_objexts_empty(struct slabobj_ext *obj_exts) {}
-static inline void mark_failed_objexts_alloc(struct slab *slab) {}
+static inline bool mark_failed_objexts_alloc(struct slab *slab) { return true; }
static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
struct slabobj_ext *vec, unsigned int objects) {}

@@ -2125,7 +2125,9 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
}
if (!vec) {
/* Mark vectors which failed to allocate */
- mark_failed_objexts_alloc(slab);
+ if (!mark_failed_objexts_alloc(slab) &&
+ slab_obj_exts(slab))
+ return 0;

return -ENOMEM;
}

--
Cheers,
Harry / Hyeonggon