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

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Date:	Wed, 18 Mar 2009 19:50:15 +0100
From:	Jan Kara <jack@...e.cz>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
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 Thu 19-03-09 00:12:22, Aneesh Kumar K.V wrote:
> 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.
  But this will zero only *new* buffers - so if they are already allocated,
get_block() won't set new flag and they won't be zeroed...
  But I'm not saying I understand why this seems to help against a corruption
under UML because we don't seem to be writing !uptodate buffers there.

									Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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