linux-kernel - RE: [PATCH] slab.h: Avoid using & for logical and of booleans

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Date:   Tue, 6 Nov 2018 10:08:26 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Bart Van Assche' <bvanassche@....org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        "Christoph Lameter" <cl@...ux.com>, Roman Gushchin <guro@...com>
Subject: RE: [PATCH] slab.h: Avoid using & for logical and of booleans

From: Bart Van Assche
> Sent: 05 November 2018 20:40
> 
> This patch suppresses the following sparse warning:
> 
> ./include/linux/slab.h:332:43: warning: dubious: x & !y
> 
> Fixes: 1291523f2c1d ("mm, slab/slub: introduce kmalloc-reclaimable caches")
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Mel Gorman <mgorman@...hsingularity.net>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Roman Gushchin <guro@...com>
> Signed-off-by: Bart Van Assche <bvanassche@....org>
> ---
>  include/linux/slab.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 918f374e7156..97d0599ddb7b 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -329,7 +329,7 @@ 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;
> +	return type_dma + is_reclaimable * !is_dma * KMALLOC_RECLAIM;

ISTM that changing is_dma and is_reclaimable from int to bool will stop the bleating.

It is also strange that this code is trying so hard here to avoid conditional instructions
and then uses several to generate the boolean values in the first place.

OTOH I'd probably write:
	int gfp_dma = 0;

#ifdef CONFIG_ZONE_DMA
	gfp_dma = __GFP_DMA;
#endif

	return flags & gfp_dma ? KMALLOC_DMA : flags & __GFP_RECLAIMABLE ? KMALLOC_RECLAIM : 0;

That might generate cmovs, but is may be better to put unlikely() around both
conditional expressions. Or redo as:

	return !unlikely(flags & (dfp_dma | __GFP_RECLAIMABLE)) ? 0 : flags & gfp_dma ? KMALLOC_DMA : KMALLOC_RECLAIM;

	David

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