Re: [RFC PATCH 07/12] mm: introduce __GFP_MIRROR to allocate mirrored pages

[Kamezawa Hiroyuki]

On 2015/06/04 22:02, Xishi Qiu wrote:
> This patch introduces a new gfp flag called "__GFP_MIRROR", it is used to
> allocate mirrored pages through buddy system.
>
> Signed-off-by: Xishi Qiu <qiuxishi@xxxxxxxxxx>

In Tony's original proposal, the motivation was to mirror all kernel memory.

Is the purpose of this patch making mirrored range available for user space ?

But, hmm... I don't think adding a new GFP flag is a good idea. It adds many conditional jumps.

How about using GFP_KERNEL for user memory if the user wants mirrored memory with mirroring
all kernel memory?

Thanks,
-Kame

> ---
>   include/linux/gfp.h | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 15928f0..89d0091 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -35,6 +35,7 @@ struct vm_area_struct;
>   #define ___GFP_NO_KSWAPD	0x400000u
>   #define ___GFP_OTHER_NODE	0x800000u
>   #define ___GFP_WRITE		0x1000000u
> +#define ___GFP_MIRROR		0x2000000u
>   /* If the above are modified, __GFP_BITS_SHIFT may need updating */
>
>   /*
> @@ -95,13 +96,15 @@ struct vm_area_struct;
>   #define __GFP_OTHER_NODE ((__force gfp_t)___GFP_OTHER_NODE) /* On behalf of other node */
>   #define __GFP_WRITE	((__force gfp_t)___GFP_WRITE)	/* Allocator intends to dirty page */
>
> +#define __GFP_MIRROR	((__force gfp_t)___GFP_MIRROR)	/* Allocate mirrored memory */
> +
>   /*
>    * This may seem redundant, but it's a way of annotating false positives vs.
>    * allocations that simply cannot be supported (e.g. page tables).
>    */
>   #define __GFP_NOTRACK_FALSE_POSITIVE (__GFP_NOTRACK)
>
> -#define __GFP_BITS_SHIFT 25	/* Room for N __GFP_FOO bits */
> +#define __GFP_BITS_SHIFT 26	/* Room for N __GFP_FOO bits */
>   #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
>
>   /* This equals 0, but use constants in case they ever change */


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