mm, slab: remove unnecessary unlikely()
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Message ID 20181104125028.3572-1-tiny.windzz@gmail.com
State In Next
Commit 230f1c3846233651c06732f874f36fcdfcbc7843
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Series
  • mm, slab: remove unnecessary unlikely()
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Commit Message

Frank Lee Nov. 4, 2018, 12:50 p.m. UTC
WARN_ON() already contains an unlikely(), so it's not necessary to use
unlikely.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
---
 mm/slab_common.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

Comments

Vlastimil Babka Nov. 5, 2018, 10:18 a.m. UTC | #1
+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@gmail.com>

Acked-by: Vlastimil Babka <vbabka@suse.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.

> ---
>  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);
>  	}
>  
>
Dmitry Vyukov Nov. 6, 2018, 5:38 a.m. UTC | #2
On Mon, Nov 5, 2018 at 11:18 AM, Vlastimil Babka <vbabka@suse.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@gmail.com>
>
> Acked-by: Vlastimil Babka <vbabka@suse.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);
>>       }
>>
>>
>
Vlastimil Babka Nov. 6, 2018, 8:54 a.m. UTC | #3
On 11/6/18 6:38 AM, Dmitry Vyukov wrote:
> On Mon, Nov 5, 2018 at 11:18 AM, Vlastimil Babka <vbabka@suse.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@gmail.com>
>>
>> Acked-by: Vlastimil Babka <vbabka@suse.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);
>>>       }
>>>
>>>
>>
David Rientjes Nov. 7, 2018, 10:55 p.m. UTC | #4
On Sun, 4 Nov 2018, Yangtao Li wrote:

> WARN_ON() already contains an unlikely(), so it's not necessary to use
> unlikely.
> 
> Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>

Acked-by: David Rientjes <rientjes@google.com>

Patch
diff mbox series

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);
 	}