Re: [PATCH 2/2] mm: slub: only warn once when allocating slab obj extensions fails

[Usama Arif <usamaarif642@gmail.com>]

On 22/05/2025 01:16, Harry Yoo wrote:
> On Tue, May 20, 2025 at 04:22:16PM +0100, Usama Arif wrote:
>>
>>
>> On 20/05/2025 15:18, Shakeel Butt wrote:
>>> On Tue, May 20, 2025 at 02:42:09PM +0100, Usama Arif wrote:
>>>>
>>>>
>>>> On 20/05/2025 14:34, Harry Yoo wrote:
>>>>> On Tue, May 20, 2025 at 01:25:47PM +0100, Usama Arif wrote:
>>>>>> In memory bound systems, a large number of warnings for failing this
>>>>>> allocation repeatedly may mask any real issues in the system
>>>>>> during memory pressure being reported in dmesg. Change this to
>>>>>> WARN_ONCE.
>>>>>>
>>>>>> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
>>>>>> Reported-by: Vlad Poenaru <vlad.wing@gmail.com>
>>>>>> Closes: https://lore.kernel.org/all/17fab2d6-5a74-4573-bcc3-b75951508f0a@gmail.com/
>>>>>> ---
>>>>>
>>>>> Hi,
>>>>>
>>>>> Please Cc SLAB ALLOCATOR folks in MAINTAINERS on patches that touch
>>>>> slab code ;)
>>>>>
>>>>
>>>> Thanks for adding them to CC! I was just thinking of this as a memory
>>>> allocation profiling issue and added the maintainers for it,
>>>> but should have added slab maintainers as well.
>>>>
>>>>
>>>>>>  mm/slub.c | 2 +-
>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/mm/slub.c b/mm/slub.c
>>>>>> index bf43c403ead2..97cb3d9e8d00 100644
>>>>>> --- a/mm/slub.c
>>>>>> +++ b/mm/slub.c
>>>>>> @@ -2102,7 +2102,7 @@ prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p)
>>>>>>  
>>>>>>  	slab = virt_to_slab(p);
>>>>>>  	if (!slab_obj_exts(slab) &&
>>>>>> -	    WARN(alloc_slab_obj_exts(slab, s, flags, false),
>>>>>> +	    WARN_ONCE(alloc_slab_obj_exts(slab, s, flags, false),
>>>>>>  		 "%s, %s: Failed to create slab extension vector!\n",
>>>>>>  		 __func__, s->name))
>>>>>
>>>>> I think this should be pr_warn_once()?
>>>>> I'm not sure why this was WARN() in the first place.
>>>>>
>>>>
>>>> Isn't WARN_ONCE the same as pr_warn_once but with needing the condition
>>>> of the first arg to be true? We only want to warn if alloc_slab_obj_exts
>>>> returns non-zero. So WARN_ONCE should be ok?
>>>>
>>>
>>> The difference is the impact on panic_on_warn users which are mostly
>>> testing bots. This warning is not actionable, so I agree with Harry to
>>> covert this to pr_warn_once().
>>>
>>
>> Sounds good! Will change it to below for the next revision.
>> Will wait for the kvmalloc conversation to conclude before sending
>> the next revision.
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 08804d2f2ead..ab0b7ee87159 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2101,11 +2101,13 @@ prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p)
>>                 return NULL;
>>  
>>         slab = virt_to_slab(p);
>> -       if (!slab_obj_exts(slab) &&
>> -           WARN(alloc_slab_obj_exts(slab, s, flags, false),
>> -                "%s, %s: Failed to create slab extension vector!\n",
>> -                __func__, s->name))
>> -               return NULL;
>> +       if (!slab_obj_exts(slab)) {
>> +               if(alloc_slab_obj_exts(slab, s, flags, false))
>> +                       pr_warn_once("%s, %s: Failed to create slab extension vector!\n",
>> +                               __func__, s->name);
>> +               else
>> +                       return NULL;
> 
> Returning NULL when alloc_slab_obj_exts() succeeds doesn't make sense.
> I think you meant something like this?
> 
> 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;
> }
> 
Yes, Thank you!

>> +       }
>>  
>>         return slab_obj_exts(slab) + obj_to_index(s, slab, p);
>>  }
>>
>