linux-ext4 - Re: [RFC][PATCH] ext3: don't read inode block if the buffer has a write error

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Date:	Mon, 23 Jun 2008 14:31:16 +0200
From:	Jan Kara <jack@...e.cz>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
	akpm@...ux-foundation.org, sct@...hat.com, adilger@....com,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
	jack@...e.cz, sugita <yumiko.sugita.yf@...achi.com>,
	Satoshi OSHIMA <satoshi.oshima.fk@...achi.com>
Subject: Re: [RFC][PATCH] ext3: don't read inode block if the buffer has a
	write error

On Mon 23-06-08 21:46:27, Nick Piggin wrote:
> On Monday 23 June 2008 21:25, Hidehiro Kawai wrote:
> > A transient I/O error can corrupt inode data.  Here is the scenario:
> >
> > (1) update inode_A at the block_B
> > (2) pdflush writes out new inode_A to the filesystem, but it results
> >     in write I/O error, at this point, BH_Uptodate flag of the buffer
> >     for block_B is cleared and BH_Write_EIO is set
> > (3) create new inode_C which located at block_B, and
> >     __ext3_get_inode_loc() tries to read on-disk block_B because the
> >     buffer is not uptodate
> > (4) if it can read on-disk block_B successfully, inode_A is
> >     overwritten by old data
> >
> > This patch makes __ext3_get_inode_loc() not read the inode block if
> > the buffer has BH_Write_EIO flag.  In this case, the buffer should
> > have the latest information, so setting the uptodate flag to the
> > buffer (this avoids WARN_ON_ONCE() in mark_buffer_dirty().)
> >
> > According to this change, we would need to test BH_Write_EIO flag for
> > the error checking.  Currently nobody checks write I/O errors on
> > metadata buffers, but it will be done in other patches I'm working on.
> 
> IMO there is a problem with all the buffer head and pagecache error
> handling in that uptodate gets cleared on write errors. This is not
> only wrong (because the in-memory copy continues to contain the most
> uptodate copy), but it will trigger assertions all over the mm and
> buffer code AFAIKS.
> 
> I don't know why it was done like this, or if anybody actually tested
> any of it, but AFAIKS the best way to fix this is to simply not
> clear any uptodate bits upon write errors.
  That would be non-trivial effort because there are lots of places which
do things like:
  wait_on_buffer(bh);
  if (!buffer_uptodate)
    /* IO error handling */

  But what you say sounds like a reasonable thing from a logical
perspective.

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