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

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Message-ID: <64b19c8f-e02e-490b-b987-9a996f36be21@gmail.com>
Date: Tue, 20 May 2025 16:22:16 +0100
From: Usama Arif <usamaarif642@...il.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>, surenb@...gle.com,
 Harry Yoo <harry.yoo@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, hannes@...xchg.org,
 vlad.wing@...il.com, linux-mm@...ck.org, kent.overstreet@...ux.dev,
 linux-kernel@...r.kernel.org, kernel-team@...a.com, vbabka@...e.cz,
 cl@...two.org, rientjes@...gle.com, roman.gushchin@...ux.dev
Subject: Re: [PATCH 2/2] mm: slub: only warn once when allocating slab obj
 extensions fails



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@...il.com>
>>>> Reported-by: Vlad Poenaru <vlad.wing@...il.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;
+       }
 
        return slab_obj_exts(slab) + obj_to_index(s, slab, p);
 }