linux-kernel - Re: [PATCH] mm, slab: remove unnecessary unlikely()

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Date:   Tue, 6 Nov 2018 09:54:49 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Yangtao Li <tiny.windzz@...il.com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, slab: remove unnecessary unlikely()

On 11/6/18 6:38 AM, Dmitry Vyukov wrote:
> On Mon, Nov 5, 2018 at 11:18 AM, Vlastimil Babka <vbabka@...e.cz> wrote:
>> +CC Dmitry
>>
>> On 11/4/18 1:50 PM, Yangtao Li wrote:
>>> WARN_ON() already contains an unlikely(), so it's not necessary to use
>>> unlikely.
>>>
>>> Signed-off-by: Yangtao Li <tiny.windzz@...il.com>
>>
>> Acked-by: Vlastimil Babka <vbabka@...e.cz>
>>
>> Maybe also change it back to WARN_ON_ONCE? I already considered it while
>> reviewing Dmitry's patch and wasn't sure. Now I think that what can
>> happen is that either a kernel bug is introduced that _ONCE is enough to
>> catch (two separate bugs introduced to both hit this would be rare, and
>> in that case the second one will be reported after the first one is
>> fixed), or this gets called with a user-supplied value, and then we want
>> to avoid spamming dmesg with multiple warnings that the user could
>> trigger at will.
> 
> 
> If you asking me, I am fine both changes.
> I was mainly interested in removing the bogus warnings that actually fire.

OK thanks. Andrew can you update the patch to WARN_ON_ONCE?

Changelog addition:
Also change WARN_ON() back to WARN_ON_ONCE() to avoid potentially
spamming dmesg with user-triggerable large allocations.

> 
>>> ---
>>>  mm/slab_common.c | 4 +---
>>>  1 file changed, 1 insertion(+), 3 deletions(-)
>>>
>>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>>> index 7eb8dc136c1c..4f54684f5435 100644
>>> --- a/mm/slab_common.c
>>> +++ b/mm/slab_common.c
>>> @@ -1029,10 +1029,8 @@ struct kmem_cache *kmalloc_slab(size_t size, gfp_t flags)
>>>
>>>               index = size_index[size_index_elem(size)];
>>>       } else {
>>> -             if (unlikely(size > KMALLOC_MAX_CACHE_SIZE)) {
>>> -                     WARN_ON(1);
>>> +             if (WARN_ON(size > KMALLOC_MAX_CACHE_SIZE))
>>>                       return NULL;
>>> -             }
>>>               index = fls(size - 1);
>>>       }
>>>
>>>
>>