linux-kernel - Re: EXT4 regression caused 4eec7

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Message-ID: <87bo8fxa92.fsf@openvz.org>
Date:	Mon, 13 May 2013 17:47:05 +0400
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>
Cc:	EUNBONG SONG <eunb.song@...sung.com>,
	"linux-ext4\@vger.kernel.org" <linux-ext4@...r.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-xfs\@vger.kernel.org" <linux-xfs@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>
Subject: Re: EXT4 regression caused 4eec7

On Mon, 13 May 2013 09:30:36 -0400, Theodore Ts'o <tytso@....edu> wrote:
> On Mon, May 13, 2013 at 03:18:09PM +0200, Jan Kara wrote:
> > Grumble. In this case I think bitfields are not worth the trouble with gcc.
> > It's a pitty we have to spend additional 8 bytes for every journal_head but
> > we'll survive... I'll send Ted a partial revert and add a comment so that
> > we won't repeat this mistake in future.
> 
> Or just switch things to use explicit 32-bit boolean operations.
> Sounds the safest way to go is to simply not trust bitfields to be
> something gcc is competent to compile correctly, and just open code it
> in standard C.  (Large portions of ext4 and e2fsprogs do this
> manually, for historical reasons, and it sounds like we have a good
> reason to do it going forward.)
> 
> Jan, Dmitry --- I still have in my tree a revert for commit 4eec708d2:
> ext4: use io_end for multiple bios, since I belive Dmitry still
> bisected a regression for xfstests 299.  Dmitry, can you confirm that
> you are definitely seeing a regression here?
Yes, this patch provoke use-after-free which detected by slab sanity checks.
>  Jan, do you mind if we
> try to figure out how to fix this during the next development cycle,
> since it was part of your much longer, extensive patch series anyway?
> 
> I've determined that the reason why I didn't see a problem was because
> xfstests 299 was failing earlier on the baseline, and crashing my
> regression tests. So I simply commented it out just so I could
> complete the testing.  It seems that xfstests 299 is problematic for
> me, and I need to focus on how to make it pass successfully.  (Dmitry,
> when I revert the commit which you identified, xfstests 299 is *still*
> failing for me....) 
In fact generic/299 always succeed for me, but it produce warning
WARNING: at fs/ext4/inode.c:3218 ext4_ext_direct_IO
and complains from slab debug. But it was missed because i've missed
this error in the logs and forget to check /proc/sys/kernel/tained.
> 
> 						- Ted
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