linux-kernel - Re: [External] Re: [PATCH 2/3] include/linux/gfp.h: use unsigned int in gfp

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Date:   Sun, 6 May 2018 11:55:32 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Huaisheng HS1 Ye <yehs1@...ovo.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "mgorman@...hsingularity.net" <mgorman@...hsingularity.net>,
        "pasha.tatashin@...cle.com" <pasha.tatashin@...cle.com>,
        "alexander.levin@...izon.com" <alexander.levin@...izon.com>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "penguin-kernel@...ove.SAKURA.ne.jp" 
        <penguin-kernel@...ove.SAKURA.ne.jp>,
        "colyli@...e.de" <colyli@...e.de>,
        NingTing Cheng <chengnt@...ovo.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [External]  Re: [PATCH 2/3] include/linux/gfp.h: use unsigned
 int in gfp_zone

On Sun, May 06, 2018 at 04:17:06PM +0000, Huaisheng HS1 Ye wrote:
> Upload my current patch and testing platform info for reference. This patch has been tested 
> on a two sockets platform.

Thank you!

> It works, but some drivers or subsystem shall be modified to fit
> these new type __GFP flags.
> They use these flags directly to realize bit manipulations like this
> below.
> 
> eg.
> swiotlb-xen.c (drivers\xen):    flags &= ~(__GFP_DMA | __GFP_HIGHMEM);
> extent_io.c (fs\btrfs):         mask &= ~(__GFP_DMA32|__GFP_HIGHMEM);
> 
> Because of these flags have been encoded within this patch, the
> above operations can cause problem.

I don't think this actually causes problems.  At least, no additional
problems.  These users will successfully clear __GFP_DMA and __GFP_HIGHMEM
no matter what values GFP_DMA and GFP_HIGHMEM have; the only problem will
be if someone calls them with a zone type they're not expecting (eg DMA32
for the first one or DMA for the second; or MOVABLE for either of them).
The thing is, they're already buggy in those circumstances.

>   */
> -#define __GFP_DMA      ((__force gfp_t)___GFP_DMA)
> -#define __GFP_HIGHMEM  ((__force gfp_t)___GFP_HIGHMEM)
> -#define __GFP_DMA32    ((__force gfp_t)___GFP_DMA32)
> +#define __GFP_DMA      ((__force gfp_t)OPT_ZONE_DMA ^ ZONE_NORMAL)
> +#define __GFP_HIGHMEM  ((__force gfp_t)ZONE_MOVABLE ^ ZONE_NORMAL)
> +#define __GFP_DMA32    ((__force gfp_t)OPT_ZONE_DMA32 ^ ZONE_NORMAL)
>  #define __GFP_MOVABLE  ((__force gfp_t)___GFP_MOVABLE)  /* ZONE_MOVABLE allowed */
[...]
>  static inline enum zone_type gfp_zone(gfp_t flags)
> {
>         enum zone_type z;
> -       int bit = (__force int) (flags & GFP_ZONEMASK);
> +       z = ((__force unsigned int)flags & ___GFP_ZONE_MASK) ^ ZONE_NORMAL;
> 
> -       z = (GFP_ZONE_TABLE >> (bit * GFP_ZONES_SHIFT)) &
> -                                        ((1 << GFP_ZONES_SHIFT) - 1);
> -       VM_BUG_ON((GFP_ZONE_BAD >> bit) & 1);
> +       if (z > OPT_ZONE_HIGHMEM) {
> +               z = OPT_ZONE_HIGHMEM +
> +                       !!((__force unsigned int)flags & ___GFP_MOVABLE);
> +       }
>         return z;
>  }

How about:

+#define __GFP_HIGHMEM  ((__force gfp_t)OPT_ZONE_HIGHMEM ^ ZONE_NORMAL)
-#define __GFP_MOVABLE  ((__force gfp_t)___GFP_MOVABLE)  /* ZONE_MOVABLE allowed */
+#define __GFP_MOVABLE  ((__force gfp_t)ZONE_MOVABLE ^ ZONE_NORMAL | \
+					___GFP_MOVABLE)

Then I think you can just make it:

static inline enum zone_type gfp_zone(gfp_t flags)
{
	return ((__force int)flags & ___GFP_ZONE_MASK) ^ ZONE_NORMAL;
}

> @@ -370,42 +368,15 @@ static inline bool gfpflags_allow_blocking(const gfp_t gfp_flags)
>  #error GFP_ZONES_SHIFT too large to create GFP_ZONE_TABLE integer
>  #endif

You should be able to delete GFP_ZONES_SHIFT too.