linux-kernel - Re: [PATCH 04/10] mm, page_alloc: Use masks and shifts when converting GFP flags to migrate types

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20150924203445.GH3009@cmpxchg.org>
Date:	Thu, 24 Sep 2015 16:34:45 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mel Gorman <mgorman@...hsingularity.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Hocko <mhocko@...nel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/10] mm, page_alloc: Use masks and shifts when
 converting GFP flags to migrate types

On Mon, Sep 21, 2015 at 11:52:36AM +0100, Mel Gorman wrote:
> @@ -14,7 +14,7 @@ struct vm_area_struct;
>  #define ___GFP_HIGHMEM		0x02u
>  #define ___GFP_DMA32		0x04u
>  #define ___GFP_MOVABLE		0x08u
> -#define ___GFP_WAIT		0x10u
> +#define ___GFP_RECLAIMABLE	0x10u
>  #define ___GFP_HIGH		0x20u
>  #define ___GFP_IO		0x40u
>  #define ___GFP_FS		0x80u
> @@ -29,7 +29,7 @@ struct vm_area_struct;
>  #define ___GFP_NOMEMALLOC	0x10000u
>  #define ___GFP_HARDWALL		0x20000u
>  #define ___GFP_THISNODE		0x40000u
> -#define ___GFP_RECLAIMABLE	0x80000u
> +#define ___GFP_WAIT		0x80000u
>  #define ___GFP_NOACCOUNT	0x100000u
>  #define ___GFP_NOTRACK		0x200000u
>  #define ___GFP_NO_KSWAPD	0x400000u
> @@ -126,6 +126,7 @@ struct vm_area_struct;
>  
>  /* This mask makes up all the page movable related flags */
>  #define GFP_MOVABLE_MASK (__GFP_RECLAIMABLE|__GFP_MOVABLE)
> +#define GFP_MOVABLE_SHIFT 3

This connects the power-of-two gfp bits to the linear migrate type
enum, so shifting back and forth between them works only with up to
two items. A hypothetical ___GFP_FOOABLE would translate to 4, not
3. I'm not expecting new migratetypes to show up anytime soon, but
this implication does not make the code exactly robust and obvious.

> @@ -152,14 +153,15 @@ struct vm_area_struct;
>  /* Convert GFP flags to their corresponding migrate type */
>  static inline int gfpflags_to_migratetype(const gfp_t gfp_flags)
>  {
> -	WARN_ON((gfp_flags & GFP_MOVABLE_MASK) == GFP_MOVABLE_MASK);
> +	VM_WARN_ON((gfp_flags & GFP_MOVABLE_MASK) == GFP_MOVABLE_MASK);
> +	BUILD_BUG_ON((1UL << GFP_MOVABLE_SHIFT) != ___GFP_MOVABLE);
> +	BUILD_BUG_ON((___GFP_MOVABLE >> GFP_MOVABLE_SHIFT) != MIGRATE_MOVABLE);
>  
>  	if (unlikely(page_group_by_mobility_disabled))
>  		return MIGRATE_UNMOVABLE;
>  
>  	/* Group based on mobility */
> -	return (((gfp_flags & __GFP_MOVABLE) != 0) << 1) |
> -		((gfp_flags & __GFP_RECLAIMABLE) != 0);
> +	return (gfp_flags & GFP_MOVABLE_MASK) >> GFP_MOVABLE_SHIFT;

I'm not sure the simplification of this line is worth the fragile
dependency between those two tables.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/