Re: [PATCH 24/27] Convert gfp_zone() to use a table ofprecalculated values

[Mel Gorman]

On Thu, Mar 19, 2009 at 12:53:34PM -0400, Christoph Lameter wrote:
> On Thu, 19 Mar 2009, Christoph Lameter wrote:
> 
> > It would work if we could check for BAD_ZONE with a VM_BUG_ON or a
> > BUILD_BUG_ON. If I get some time I will look into this.
> 
> Here is such a patch. Boots on my machine and working with that kernel
> now. There is a slight gcc problem in that the table is likely repeated
> for each compilation unit. Anyone know how to fix that?
> 

I ran into exactly that problem and ended up shoving the table into
page_alloc.c but then there is no benefits from having the table statically
declared because there is no constant folding.

Just to confirm: With your patch, gfp_zone_table() does end up in different
complation units

$ readelf -s vmlinux | grep gfp_zone_table
  5479: c03a9ea0    64 OBJECT  LOCAL  DEFAULT    5 gfp_zone_table
  5537: c03a9f20    64 OBJECT  LOCAL  DEFAULT    5 gfp_zone_table
  5753: c03a9fe0    64 OBJECT  LOCAL  DEFAULT    5 gfp_zone_table

> Subject: Use a table lookup for gfp_zone and check for errors in flags passed to the page allocator
> 
> Use a table to lookup the zone to use given gfp_flags using gfp_zone().
> 
> This simplifies the code in gfp_zone() and also keeps the ability of the compiler to
> use constant folding to get rid of gfp_zone processing.
> 
> One problem with this patch is that we define a static const array in gfp.h. This results
> in every compilation unit to reserve its own space for the array. There must be some
> trick to get the compiler to allocate this only once. The contents of the array
> must be described in the header file otherwise the compiler will not be able to
> determine the value of a lookup in the table.
> 

Yep, that is exactly the problem I hit but I didn't find a suitable answer.

> Signed-off-by: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
> 
> Index: linux-2.6/include/linux/gfp.h
> ===================================================================
> --- linux-2.6.orig/include/linux/gfp.h	2009-03-19 11:43:32.000000000 -0500
> +++ linux-2.6/include/linux/gfp.h	2009-03-19 11:48:38.000000000 -0500
> @@ -19,7 +19,8 @@
>  #define __GFP_DMA	((__force gfp_t)0x01u)
>  #define __GFP_HIGHMEM	((__force gfp_t)0x02u)
>  #define __GFP_DMA32	((__force gfp_t)0x04u)
> -
> +#define __GFP_MOVABLE	((__force gfp_t)0x08u)  /* Page is movable */
> +#define GFP_ZONEMASK	((__force gfp_t)0x0fu)

To avoid magic number syndrome, you could define GFP_ZONEMASK as

	(__GFP_DMA | __GFP_HIGHMEM | __GFP_DMA32 | __GFP_MOVABLE)

>  /*
>   * Action modifiers - doesn't change the zoning
>   *
> @@ -49,7 +50,6 @@
>  #define __GFP_HARDWALL   ((__force gfp_t)0x20000u) /* Enforce hardwall cpuset memory allocs */
>  #define __GFP_THISNODE	((__force gfp_t)0x40000u)/* No fallback, no policies */
>  #define __GFP_RECLAIMABLE ((__force gfp_t)0x80000u) /* Page is reclaimable */
> -#define __GFP_MOVABLE	((__force gfp_t)0x100000u)  /* Page is movable */
> 
>  #define __GFP_BITS_SHIFT 21	/* Room for 21 __GFP_FOO bits */
>  #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
> @@ -111,24 +111,56 @@
>  		((gfp_flags & __GFP_RECLAIMABLE) != 0);
>  }
> 
> -static inline enum zone_type gfp_zone(gfp_t flags)
> -{
> +#ifdef CONFIG_ZONE_HIGHMEM
> +#define OPT_ZONE_HIGHMEM ZONE_HIGHMEM
> +#else
> +#define OPT_ZONE_HIGHMEM ZONE_NORMAL
> +#endif
> +
>  #ifdef CONFIG_ZONE_DMA
> -	if (flags & __GFP_DMA)
> -		return ZONE_DMA;
> +#define OPT_ZONE_DMA ZONE_DMA
> +#else
> +#define OPT_ZONE_DMA ZONE_NORMAL
>  #endif
> +
>  #ifdef CONFIG_ZONE_DMA32
> -	if (flags & __GFP_DMA32)
> -		return ZONE_DMA32;
> +#define OPT_ZONE_DMA32 ZONE_DMA32
> +#else
> +#define OPT_ZONE_DMA32 OPT_ZONE_DMA
>  #endif
> -	if ((flags & (__GFP_HIGHMEM | __GFP_MOVABLE)) ==
> -			(__GFP_HIGHMEM | __GFP_MOVABLE))
> -		return ZONE_MOVABLE;
> -#ifdef CONFIG_HIGHMEM
> -	if (flags & __GFP_HIGHMEM)
> -		return ZONE_HIGHMEM;
> +
> +#define BAD_ZONE MAX_NR_ZONES
> +
> +static const enum zone_type gfp_zone_table[GFP_ZONEMASK + 1] = {
> +	ZONE_NORMAL,		/* 00 No flags set */
> +	OPT_ZONE_DMA,		/* 01 GFP_DMA */
> +	OPT_ZONE_HIGHMEM,	/* 02 GFP_HIGHMEM */
> +	BAD_ZONE,		/* 03 GFP_HIGHMEM GFP_DMA */
> +	OPT_ZONE_DMA32,		/* 04 GFP_DMA32 */
> +	BAD_ZONE,		/* 05 GFP_DMA32 GFP_DMA */
> +	BAD_ZONE,		/* 06 GFP_DMA32 GFP_HIGHMEM */
> +	BAD_ZONE,		/* 07 GFP_DMA32 GFP_HIGHMEM GFP_DMA */
> +	ZONE_NORMAL,		/* 08 ZONE_MOVABLE */
> +	OPT_ZONE_DMA,		/* 09 MOVABLE + DMA */
> +	ZONE_MOVABLE,		/* 0A MOVABLE + HIGHMEM */
> +	BAD_ZONE,		/* 0B MOVABLE + HIGHMEM + DMA */
> +	OPT_ZONE_DMA32,		/* 0C MOVABLE + DMA32 */
> +	BAD_ZONE,		/* 0D MOVABLE + DMA32 + DMA */
> +	BAD_ZONE,		/* 0E MOVABLE + DMA32 + HIGHMEM */
> +	BAD_ZONE		/* 0F MOVABLE + DMA32 + HIGHMEM + DMA */
> +};
> +
> +static inline enum zone_type gfp_zone(gfp_t flags)
> +{
> +	enum zone_type zone = gfp_zone_table[flags & 0xf];
> +

flags & GFP_ZONEMASK here

> +	if (__builtin_constant_p(zone))
> +		BUILD_BUG_ON(zone == BAD_ZONE);
> +#ifdef CONFIG_DEBUG_VM
> +	else
> +		BUG_ON(zone == BAD_ZONE);
>  #endif

That could be made a bit prettier with

	if (__builtin_constant_p(zone))
		BUILD_BUG_ON(zone == BAD_ZONE);
	VM_BUG_ON(zone == BAD_ZONE);

> -	return ZONE_NORMAL;
> +	return zone;
>  }
> 
>  /*
> Index: linux-2.6/include/linux/mmzone.h
> ===================================================================
> --- linux-2.6.orig/include/linux/mmzone.h	2009-03-19 11:47:00.000000000 -0500
> +++ linux-2.6/include/linux/mmzone.h	2009-03-19 11:47:54.000000000 -0500
> @@ -240,7 +240,8 @@
>  	ZONE_HIGHMEM,
>  #endif
>  	ZONE_MOVABLE,
> -	__MAX_NR_ZONES
> +	__MAX_NR_ZONES,
> +	BAD_ZONE
>  };
> 
>  #ifndef __GENERATING_BOUNDS_H
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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