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

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Date:	Wed, 18 Mar 2009 15:13:31 +0100
From:	Jan Kara <jack@...e.cz>
To:	Nick Piggin <npiggin@...e.de>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 3/4] fs: Avoid data corruption with blocksize < pagesize

On Wed 18-03-09 13:00:23, Nick Piggin 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
> > writepage() is called on the page
> >   block_write_full_page() writes buffers with garbage
> > 
> > This patch fixes the problem by skipping !uptodate buffers in
> > block_write_full_page().
> > 
> > CC: Nick Piggin <npiggin@...e.de>
> > Signed-off-by: Jan Kara <jack@...e.cz>
> > ---
> >  fs/buffer.c |    7 ++++++-
> >  1 files changed, 6 insertions(+), 1 deletions(-)
> > 
> > diff --git a/fs/buffer.c b/fs/buffer.c
> > index 9f69741..22c0144 100644
> > --- a/fs/buffer.c
> > +++ b/fs/buffer.c
> > @@ -1774,7 +1774,12 @@ static int __block_write_full_page(struct inode *inode, struct page *page,
> >  	} while (bh != head);
> >  
> >  	do {
> > -		if (!buffer_mapped(bh))
> > +		/*
> > +		 * Parallel write could have already mapped the buffers but
> > +		 * it then had to restart before copying in new data. We
> > +		 * must avoid writing garbage so just skip the buffer.
> > +		 */
> > +		if (!buffer_mapped(bh) || !buffer_uptodate(bh))
> >  			continue;
> 
> I don't quite see how this can happen. Further down in this loop,
> we do a test_clear_buffer_dirty(), which should exclude this I
> think? And marking the buffer dirty if it is not uptodate should
> be a bug.
  Hmm, this patch definitely does something important because without it I
hit corruption in UML in ~20 minutes and with it no corruption happens
in ~3 hours. Maybe someone calls set_page_dirty() on the page and
__set_page_dirty_buffers() unconditionally dirties all the buffers the
page has? But I still don't see how the write could be lost which is what
I observe in fsx-linux test. I'm doing some more tests to understand this
better.

									Honza

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