linux-kernel - Re: [PATCH] Fix memory hotplug and bouncing in block layer

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Date:	Sat, 08 Mar 2008 14:33:45 +0900
From:	Yasunori Goto <y-goto@...fujitsu.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	axboe@...nel.dk, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix memory hotplug and bouncing in block layer


Oh, thanks for your fixing.

Acked-by: Yasunori Goto <y-goto@...fujitsu.com>


> Fix memory hotplug and bouncing in block layer
> 
> Only noticed this while hacking something else, no test case.
> 
> blk_max_low_pfn is initialized once at bootup by the block layer
> from max_low_pfn. But max_low_pfn is not necessarily constant
> over the runtime of the system when you consider memory hotplug.
> What could happen if that someone adds memory later the block
> layer wouldn't get updated and then start bouncing memory unnecessarily.
> 
> Also on 64bit blk_max_low_pfn actually isn't needed because
> it just disables bouncing essentially and there is no highmem.
> And nobody can pass pfns > max_low_pfn to the block layer, 
> because those wouldn't have a struct page and I suspect
> block layer wouldn't be very happy without that.
> 
> So set BLK_BOUNCE_HIGH to infinity (-1ULL) on 64bit. That
> avoids the problem of having to update it on memory hotadd.
> 
> On 32bit I kept the same behaviour because at least on i386
> memory hotadd only adds HIGHMEM, never lowmem.
> 
> BLK_BOUNCE_ANY is always set to infinity on both 32 and 64bit.
> 
> Signed-off-by: Andi Kleen <ak@...e.de>
> 
> ---
>  include/linux/blkdev.h |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> Index: linux/include/linux/blkdev.h
> ===================================================================
> --- linux.orig/include/linux/blkdev.h
> +++ linux/include/linux/blkdev.h
> @@ -532,8 +532,13 @@ extern unsigned long blk_max_low_pfn, bl
>   * BLK_BOUNCE_ANY	: don't bounce anything
>   * BLK_BOUNCE_ISA	: bounce pages above ISA DMA boundary
>   */
> +
> +#if BITS_PER_LONG == 32
>  #define BLK_BOUNCE_HIGH		((u64)blk_max_low_pfn << PAGE_SHIFT)
> -#define BLK_BOUNCE_ANY		((u64)blk_max_pfn << PAGE_SHIFT)
> +#else
> +#define BLK_BOUNCE_HIGH		-1ULL
> +#endif
> +#define BLK_BOUNCE_ANY		(-1ULL)
>  #define BLK_BOUNCE_ISA		(ISA_DMA_THRESHOLD)

-- 
Yasunori Goto 


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