linux-kernel - Re: [PATCH] mm/slab: use IS_ENABLED() instead of ZONE_DMA

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Date:	Thu, 25 Sep 2014 14:50:47 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Paul Bolle <pebolle@...cali.nl>
Cc:	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@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/slab: use IS_ENABLED() instead of ZONE_DMA_FLAG

On Thu, Sep 25, 2014 at 07:57:31PM +0200, Paul Bolle wrote:
> The Kconfig symbol ZONE_DMA_FLAG probably predates the introduction of
> IS_ENABLED(). Remove it and replace its two uses with the equivalent
> IS_ENABLED(CONFIG_ZONE_DMA).
> 
> Signed-off-by: Paul Bolle <pebolle@...cali.nl>
> ---
> Build tested on x86_64 (on top of next-20140925).
> 
> Run tested on i686 (on top of v3.17-rc6). That test required me to
> switch from SLUB (Fedora's default) to SLAB. That makes running this
> patch both more scary and less informative. Besides, I have no idea how
> to hit the codepaths I just changed. You'd expect this to not actually
> change slab.o, but I'm not sure how to check that. So, in short: review
> very much appreciated.
> 
>  mm/Kconfig | 5 -----
>  mm/slab.c  | 4 ++--
>  2 files changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 886db21..8e860c7 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -273,11 +273,6 @@ config ARCH_ENABLE_HUGEPAGE_MIGRATION
>  config PHYS_ADDR_T_64BIT
>  	def_bool 64BIT || ARCH_PHYS_ADDR_T_64BIT
>  
> -config ZONE_DMA_FLAG
> -	int
> -	default "0" if !ZONE_DMA
> -	default "1"
> -
>  config BOUNCE
>  	bool "Enable bounce buffers"
>  	default y
> diff --git a/mm/slab.c b/mm/slab.c
> index 628f2b5..766c90e 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -2243,7 +2243,7 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
>  	cachep->freelist_size = freelist_size;
>  	cachep->flags = flags;
>  	cachep->allocflags = __GFP_COMP;
> -	if (CONFIG_ZONE_DMA_FLAG && (flags & SLAB_CACHE_DMA))
> +	if (IS_ENABLED(CONFIG_ZONE_DMA) && (flags & SLAB_CACHE_DMA))
>  		cachep->allocflags |= GFP_DMA;

GFP_DMA is actually safe to use even without CONFIG_ZONE_DMA, so you
only need to check for SLAB_CACHE_DMA here.

> @@ -2516,7 +2516,7 @@ static void cache_init_objs(struct kmem_cache *cachep,
>  
>  static void kmem_flagcheck(struct kmem_cache *cachep, gfp_t flags)
>  {
> -	if (CONFIG_ZONE_DMA_FLAG) {
> +	if (IS_ENABLED(CONFIG_ZONE_DMA)) {
>  		if (flags & GFP_DMA)
>  			BUG_ON(!(cachep->allocflags & GFP_DMA));
>  		else

I think this assertion can be removed altogether and replaced by ORing
the passed in flags with the cache gfp flags.  The page allocator will
catch any contradictions, but the 3 callsites that actually do use DMA
caches are well-behaved as of now.
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