linux-ext4 - Re: Odd "leak" of extent info into data blocks?

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Message-ID: <20090908194045.GQ22901@mit.edu>
Date:	Tue, 8 Sep 2009 15:40:45 -0400
From:	Theodore Tso <tytso@....edu>
To:	Curt Wohlgemuth <curtw@...gle.com>
Cc:	Valerie Aurora <vaurora@...hat.com>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: Odd "leak" of extent info into data blocks?

On Tue, Sep 08, 2009 at 11:21:11AM -0700, Curt Wohlgemuth wrote:
> Hi Valerie:
> 
> On Tue, Sep 8, 2009 at 10:56 AM, Valerie Aurora<vaurora@...hat.com> wrote:
> > Hey, did you figure this out?  If not, I want to have a bug open
> > somewhere.
> 
> Yes, sorry.  I was going to post a patch for this, but have been
> waiting to verify that it really fixes the issue.  And see the thread
> started by Frank Mayhar about fsync issues as well...
> 
> The problem is a race, between the last write to a to-be-freed
> metadata block (to update the extent header) and the block being
> marked free in the on-disk/buddy bitmaps.  Note that this only happens
> without a journal, since *with* a journal the ordering is done
> correctly.

Just to clarify, this a race that shows up even without an unclean
shutdown, right?

> Without a journal, the block buffer_head is written to, the
> buffer_head is marked dirty, and the bitmaps are updated via
> ext4_free_blocks().  In rare cases, the block is re-allocated for
> another inode and written to -- subsequently, the writeback mechanism
> will then flush the dirty extent header back to disk.  That's why it
> looks like "leaked extent data" in the data block.

If this shows up even without an unclean shutdown, then it sounds like
the problem is a missing bforget() call.

					- Ted
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