linux-kernel - Re: [PATCH] SLUB: Fix __GFP

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Message-ID: <4B0D77CC.3020807@cs.helsinki.fi>
Date:	Wed, 25 Nov 2009 20:30:36 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Joe Perches <joe@...ches.com>
CC:	linux-kernel@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [PATCH] SLUB: Fix __GFP_ZERO unlikely() annotation

Joe Perches wrote:
> On Wed, 2009-11-25 at 20:21 +0200, Pekka Enberg wrote:
>> The unlikely() annotation in slab_alloc() covers too much of the expression.
>> It's actually very likely that the object is not NULL so use unlikely() only
>> for the __GFP_ZERO expression like SLAB does.
> []
>> +++ b/mm/slub.c
>> @@ -1735,7 +1735,7 @@ static __always_inline void *slab_alloc(struct kmem_cache *s,
>> -	if (unlikely((gfpflags & __GFP_ZERO) && object))
>> +	if (unlikely(gfpflags & __GFP_ZERO) && object)
>>  		memset(object, 0, objsize);
> 
> so why not use
> 
> 	if (unlikely(gfpflags & __GFP_ZERO) && likely(object))

Because that has no effect on the generated code.
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