* [PATCH] slab: Fix obj_ext is mistakenly considered NULL due to race condition
@ 2025-10-23 1:21 Hao Ge
2025-10-23 2:24 ` Harry Yoo
0 siblings, 1 reply; 8+ messages in thread
From: Hao Ge @ 2025-10-23 1:21 UTC (permalink / raw)
To: Vlastimil Babka, Andrew Morton, Christoph Lameter,
David Rientjes, Roman Gushchin, Harry Yoo, Suren Baghdasaryan
Cc: Shakeel Butt, linux-mm, linux-kernel, Hao Ge
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;
+ }
}
return slab_obj_exts(slab) + obj_to_index(s, slab, p);
--
2.25.1
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH] slab: Fix obj_ext is mistakenly considered NULL due to race condition
2025-10-23 1:21 [PATCH] slab: Fix obj_ext is mistakenly considered NULL due to race condition Hao Ge
@ 2025-10-23 2:24 ` Harry Yoo
2025-10-23 3:11 ` Hao Ge
0 siblings, 1 reply; 8+ messages in thread
From: Harry Yoo @ 2025-10-23 2:24 UTC (permalink / raw)
To: Hao Ge
Cc: Vlastimil Babka, Andrew Morton, Christoph Lameter,
David Rientjes, Roman Gushchin, Suren Baghdasaryan, Shakeel Butt,
linux-mm, linux-kernel, Hao Ge
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.
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;
}
--
Cheers,
Harry / Hyeonggon
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH] slab: Fix obj_ext is mistakenly considered NULL due to race condition
2025-10-23 2:24 ` Harry Yoo
@ 2025-10-23 3:11 ` Hao Ge
2025-10-23 7:50 ` Harry Yoo
0 siblings, 1 reply; 8+ messages in thread
From: Hao Ge @ 2025-10-23 3:11 UTC (permalink / raw)
To: Harry Yoo
Cc: Vlastimil Babka, Andrew Morton, Christoph Lameter,
David Rientjes, Roman Gushchin, Suren Baghdasaryan, Shakeel Butt,
linux-mm, linux-kernel, Hao Ge
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;
> }
>
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH] slab: Fix obj_ext is mistakenly considered NULL due to race condition
2025-10-23 3:11 ` Hao Ge
@ 2025-10-23 7:50 ` Harry Yoo
2025-10-23 8:23 ` Hao Ge
0 siblings, 1 reply; 8+ messages in thread
From: Harry Yoo @ 2025-10-23 7:50 UTC (permalink / raw)
To: Hao Ge
Cc: Vlastimil Babka, Andrew Morton, Christoph Lameter,
David Rientjes, Roman Gushchin, Suren Baghdasaryan, Shakeel Butt,
linux-mm, linux-kernel, Hao Ge
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.
> > But anyway, I think there is a better way to do this:
What do you think about the diff I suggested below, though?
> > 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;
> > }
> >
>
--
Cheers,
Harry / Hyeonggon
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH] slab: Fix obj_ext is mistakenly considered NULL due to race condition
2025-10-23 7:50 ` Harry Yoo
@ 2025-10-23 8:23 ` Hao Ge
2025-10-23 8:46 ` Hao Ge
0 siblings, 1 reply; 8+ messages in thread
From: Hao Ge @ 2025-10-23 8:23 UTC (permalink / raw)
To: Harry Yoo
Cc: Vlastimil Babka, Andrew Morton, Christoph Lameter,
David Rientjes, Roman Gushchin, Suren Baghdasaryan, Shakeel Butt,
linux-mm, linux-kernel, Hao Ge
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;
>>> }
>>>
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH] slab: Fix obj_ext is mistakenly considered NULL due to race condition
2025-10-23 8:23 ` Hao Ge
@ 2025-10-23 8:46 ` Hao Ge
2025-10-23 9:06 ` Harry Yoo
0 siblings, 1 reply; 8+ messages in thread
From: Hao Ge @ 2025-10-23 8:46 UTC (permalink / raw)
To: Harry Yoo
Cc: Vlastimil Babka, Andrew Morton, Christoph Lameter,
David Rientjes, Roman Gushchin, Suren Baghdasaryan, Shakeel Butt,
linux-mm, linux-kernel, Hao Ge
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;
>>>> }
>>>>
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH] slab: Fix obj_ext is mistakenly considered NULL due to race condition
2025-10-23 8:46 ` Hao Ge
@ 2025-10-23 9:06 ` Harry Yoo
2025-10-23 9:11 ` Hao Ge
0 siblings, 1 reply; 8+ messages in thread
From: Harry Yoo @ 2025-10-23 9:06 UTC (permalink / raw)
To: Hao Ge
Cc: Vlastimil Babka, Andrew Morton, Christoph Lameter,
David Rientjes, Roman Gushchin, Suren Baghdasaryan, Shakeel Butt,
linux-mm, linux-kernel, Hao Ge
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?
> > > > > 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;
> > > > > }
> > > > >
--
Cheers,
Harry / Hyeonggon
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH] slab: Fix obj_ext is mistakenly considered NULL due to race condition
2025-10-23 9:06 ` Harry Yoo
@ 2025-10-23 9:11 ` Hao Ge
0 siblings, 0 replies; 8+ messages in thread
From: Hao Ge @ 2025-10-23 9:11 UTC (permalink / raw)
To: Harry Yoo
Cc: Vlastimil Babka, Andrew Morton, Christoph Lameter,
David Rientjes, Roman Gushchin, Suren Baghdasaryan, Shakeel Butt,
linux-mm, linux-kernel, Hao Ge
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;
>>>>>> }
>>>>>>
^ permalink raw reply [flat|nested] 8+ messages in thread
end of thread, other threads:[~2025-10-23 9:12 UTC | newest]
Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
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2025-10-23 1:21 [PATCH] slab: Fix obj_ext is mistakenly considered NULL due to race condition Hao Ge
2025-10-23 2:24 ` Harry Yoo
2025-10-23 3:11 ` Hao Ge
2025-10-23 7:50 ` Harry Yoo
2025-10-23 8:23 ` Hao Ge
2025-10-23 8:46 ` Hao Ge
2025-10-23 9:06 ` Harry Yoo
2025-10-23 9:11 ` Hao Ge
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