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

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Date:   Tue, 6 Nov 2018 06:38:08 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
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 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.


>> ---
>>  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);
>>       }
>>
>>
>