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Message-ID: <abc7bc71-c9a4-4a19-a47f-f6d6f40608fb@linux.dev>
Date: Thu, 23 Oct 2025 16:46:42 +0800
From: Hao Ge <hao.ge@...ux.dev>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>,
Andrew Morton <akpm@...ux-foundation.org>, Christoph Lameter
<cl@...two.org>, David Rientjes <rientjes@...gle.com>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Suren Baghdasaryan <surenb@...gle.com>, Shakeel Butt
<shakeel.butt@...ux.dev>, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Hao Ge <gehao@...inos.cn>
Subject: Re: [PATCH] slab: Fix obj_ext is mistakenly considered NULL due to
race condition
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@...inos.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@...inos.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;
>>>> }
>>>>
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