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

Hao Ge
2025-10-23T08:24:42Z

Hi Harry

On 2025/10/23 15:50, Harry Yoo wrote:
> On Thu, Oct 23, 2025 at 11:11:56AM +0800, Hao Ge wrote:
>> 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.
> Oh, just to be clear I was talking about the other warning:
> pr_warn_once("%s, %s: Failed to create slab extension vector!", __func__, s->name);
>
>> 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 yeah I see what you mean.
>
> As you mentioned, if the process that failed to allocate the vector wins
> cmpxchg(), later process that successfully allocate the vector would
> call set_codetag_empty(), so no warning.
>
> But if the process that allocates the vector wins cmpxchg(),
> then it won't call set_codetag_empty(), so the process
> that was trying to set OBJEXTS_ALLOC_FAIL now needs to set the tag.

Yes, the case I'm encountering is exactly this one.

>
>>> But anyway, I think there is a better way to do this:
> What do you think about the diff I suggested below, though?

Sorry for the delayed response earlier; I was trying to deduce all
possible scenarios.

It makes sense to me, and I will submit the V2 version based on this
suggestion.

Thank you for your help.

>
>>> 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;
>>> }
>>>