linux-ext4 - Re: [PATCH 3/4] fs: Avoid data corruption with blocksize

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Date:	Thu, 19 Mar 2009 00:12:22 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Jan Kara <jack@...e.cz>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-ext4@...r.kernel.org,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 3/4] fs: Avoid data corruption with blocksize < pagesize

On Tue, Mar 17, 2009 at 06:33:54PM +0100, Jan Kara wrote:
> Assume the following situation:
> Filesystem with blocksize < pagesize - suppose blocksize = 1024,
> pagesize = 4096. File 'f' has first four blocks already allocated.
> (line with "state:" contains the state of buffers in the page - m = mapped,
> u = uptodate, d = dirty)
> 
>   process 1:                       process 2:
> 
> write to 'f' bytes 0 - 1024
>   state: |mud,-,-,-|, page dirty
>                                    write to 'f' bytes 1024 - 4096:
>                                      __block_prepare_write() maps blocks
>                                        state: |mud,m,m,m|, page dirty
>                                      we fail to copy data -> copied = 0
>                                      block_write_end() does nothing
>                                      page gets unlocked


If copied = 0 then in  block_write_end we do

page_zero_new_buffers(page, start+copied, start+len

which would mean we should not see garbage.


> writepage() is called on the page
>   block_write_full_page() writes buffers with garbage
> 

-aneesh
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