linux-ext4 - Re: Apparent serious progressive ext4 data corruption bug in 3.6.3 (and other stable branches?)

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Date:	Fri, 26 Oct 2012 16:56:18 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Nix <nix@...eri.org.uk>
Cc:	Eric Sandeen <sandeen@...hat.com>, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Bryan Schumaker <bjschuma@...app.com>,
	Peng Tao <bergwolf@...il.com>, Trond.Myklebust@...app.com,
	gregkh@...uxfoundation.org, linux-nfs@...r.kernel.org
Subject: Re: Apparent serious progressive ext4 data corruption bug in 3.6.3
 (and other stable branches?)

On Fri, Oct 26, 2012 at 09:37:08PM +0100, Nix wrote:
> 
> I can reproduce this on a small filesystem and stick the image somewhere
> if that would be of any use to anyone. (If I'm very lucky, merely making
> this offer will make the problem go away. :} )

I'm not sure the image is going to be that useful.  What we really
need to do is to get a reliable reproduction of what _you_ are seeing.

It's clear from Eric's experiments that journal_checksum is dangerous.
In fact, I will likely put it under an #ifdef EXT4_EXPERIMENTAL to try
to discourage people from using it in the future.  There are things
I've been planning on doing to make it be safer, but there's a very
good *reason* that both journal_checksum and journal_async_commit are
not on by default.

That's why one of the things I asked you to do when you had time was
to see if you could reproduce the problem you are seeing w/o
nobarrier,journal_checksum,journal_async_commit.

The other experiment that would be really useful if you could do is to
try to apply these two patches which I sent earlier this week:

[PATCH 1/2] ext4: revert "jbd2: don't write superblock when if its empty
[PATCH 2/2] ext4: fix I/O error when unmounting an ro file system

... and see if they make a difference.

If they don't make a difference, I don't want to apply patches just
for placebo/PR reasons.  And for Eric at least, he can reproduce the
journal checksum error followed by fairly significant corruption
reported by e2fsck with journal_checksum, and the presence or absense
of these patches make no difference for him.  So I really don't want
to push these patches to Linus until I get confirmation that they make
a difference to *somebody*.

Regards,

						- Ted
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