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 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))

if another process that allocates the vector, we will lose one count.


>>>>    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;
>>>>        }
>>>>