linux-kernel - Re: [PATCH] Suppress the sparse warning ./include/linux/slab.h:332:43: warning: dubious: x & !y

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Date:   Wed, 14 Nov 2018 14:25:05 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     "Darryl T. Agostinelli" <dagostinelli@...il.com>
cc:     linux-mm@...ck.org, cl@...ux.com, bvanassche@....org,
        akpm@...ux-foundation.org, penberg@...nel.org, vbabka@...e.cz,
        iamjoonsoo.kim@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Suppress the sparse warning ./include/linux/slab.h:332:43:
 warning: dubious: x & !y

On Thu, 8 Nov 2018, Darryl T. Agostinelli wrote:

> Signed-off-by: Darryl T. Agostinelli <dagostinelli@...il.com>
> ---
>  include/linux/slab.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 918f374e7156..883b7f56bf35 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -317,6 +317,7 @@ static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
>  	int is_dma = 0;
>  	int type_dma = 0;
>  	int is_reclaimable;
> +	int y;
>  
>  #ifdef CONFIG_ZONE_DMA
>  	is_dma = !!(flags & __GFP_DMA);
> @@ -329,7 +330,10 @@ static __always_inline enum kmalloc_cache_type kmalloc_type(gfp_t flags)
>  	 * If an allocation is both __GFP_DMA and __GFP_RECLAIMABLE, return
>  	 * KMALLOC_DMA and effectively ignore __GFP_RECLAIMABLE
>  	 */
> -	return type_dma + (is_reclaimable & !is_dma) * KMALLOC_RECLAIM;
> +
> +	y = (is_reclaimable & (is_dma == 0 ? 1 : 0));
> +
> +	return type_dma + y * KMALLOC_RECLAIM;
>  }
>  
>  /*

I agree with you that the function as written is less than pretty :)  How 
does the assembly change as a result of this code change, however?  This 
will be in the kmalloc() path so impacting the assembly to fix a sparse 
warning may not be warranted.