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

[Usama Arif <usamaarif642@gmail.com>]

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 coding style guide explicitly states that:
>> Do not WARN lightly
>> ===================
>>
>> WARN*() is intended for unexpected, this-should-never-happen situations.
>> WARN*() macros are not to be used for anything that is expected to happen
>> during normal operation. These are not pre- or post-condition asserts,
>> for example. Again: WARN*() must not be used for a condition that is
>> expected to trigger easily, for example, by user space actions.
>> pr_warn_once() is a possible alternative, if you need to notify the user
>> of a problem.
> 
> And failing to allocate the extension vector can happen during normal
> operations.
> 
> panic_on_warn users will be unhappy if they notice their kernel panicked
> just because their kernel failed to allocate slab extension vectors, which is
> a totally normal situtation.
> 
>>  		return NULL;
>> -- 
>> 2.47.1
>>
>>
>