Subject: [PATCH v2 1/1] block: blk-merge: don't merge the pages with non-contiguous descriptors

From: [email-masked] (Subhash Jadavani)
Date: Wed, 16 Jan 2013 18:09:14 +0530

On 1/16/2013 4:02 PM, James Bottomley wrote:
> On Wed, 2013-01-16 at 12:07 +0530, Subhash Jadavani wrote:
>
>> Now consider this call stack from MMC block driver (this is on the ARmv7
>> based board):
>>       [   98.918174] [<c001b50c>] (v7_dma_inv_range+0x30/0x48) from
>> [<c0017b8c>] (dma_cache_maint_page+0x1c4/0x24c)
>>       [   98.927819] [<c0017b8c>] (dma_cache_maint_page+0x1c4/0x24c) from
>> [<c0017c28>] (___dma_page_cpu_to_dev+0x14/0x1c)
>>       [   98.937982] [<c0017c28>] (___dma_page_cpu_to_dev+0x14/0x1c) from
>> [<c0017ff8>] (dma_map_sg+0x3c/0x114)
> OK, so this is showing that ARM itself is making the assumption that the
> pages are contiguous in the page offset map.
>
> Fix this by doing the increment via the pfn, which will do the right
> thing whatever the memory model.
>
> Signed-off-by: James Bottomley <[email-masked]>

Thanks James. Yes, it make sense to fix the ARM code itself if it is the 
only one giving this trouble.
I have tried your change below and it also fixes this issue (without 
having my blk-merge patch). I will forward your change to Russel King to 
see what he thinks about it.

Regards,
Subhash
>
> ---
>
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index 6b2fb87..ab88c5b 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -809,7 +809,7 @@ static void dma_cache_maint_page(struct page *page, unsigned long offset,
>   			op(vaddr, len, dir);
>   		}
>   		offset = 0;
> -		page++;
> +		page = pfn_to_page(page_to_pfn(page) + 1);
>   		left -= len;
>   	} while (left);
>   }
>
>
> --
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