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

[Hao Ge <hao.ge@linux.dev>]

Hi Harry


On 2025/10/23 17:06, Harry Yoo wrote:
> On Thu, Oct 23, 2025 at 04:46:42PM +0800, Hao Ge wrote:
>> Hi Harry
>>
>>
>> On 2025/10/23 16:23, Hao Ge wrote:
>>> 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; }
>> Maybe it returns false here.
>>
>> When CONFIG_MEM_ALLOC_PROFILING_DEBUG is not enabled,
>>
>> The following condition will never be executed:
>>
>> if (!mark_failed_objexts_alloc(slab) && slab_obj_exts(slab))
> Good point. But without CONFIG_MEM_ALLOC_PROFILING_DEBUG, we don't know
> if someone else successfully allocated the vector or not (unlike, with
> CONFIG_MEM_ALLOC_PROFILING_DEBUG enabled, we know that when we lose
> cmpxchg()). We cannot "fix" the case where a process fails to allocate
> the vector but another allocates the vector.
>
> So I'm not sure if checking slab_obj_exts() once more is worth it in
> this case, but I'm fine with either way.
>
>> if another process that allocates the vector, we will lose one count.
> By "one count" you mean skipping accounting the object in memory
> profiling, right?
Yes.
>
>>>>>>     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;
>>>>>>         }
>>>>>>