linux-kernel - 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 19:17:33 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>, 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>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH] ext3: don't read inode block if the buffer has a
 write error

On Mon, 23 Jun 2008 21:46:27 +1000 Nick Piggin <nickpiggin@...oo.com.au> 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.

There's a plausible-sounding reason for this behaviour which I forgot
about three years ago.  Maybe Linus remembers?

Meanwhile I guess I'll queue the patch, sad though it is.
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