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

Hao Ge
2025-10-23T03:13:00Z

Hi Harry

On 2025/10/23 10:24, Harry Yoo wrote:
> 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.

The process that successfully allocates slab_exts will call
handle_failed_objexts_alloc, setting ref->ct = CODETAG_EMPTY

to prevent the warning from being triggered.

> 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;
> }
>