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

[Eric Sandeen <sandeen@redhat.com>]

On 10/24/2012 12:23 AM, Theodore Ts'o wrote:
> On Tue, Oct 23, 2012 at 11:27:09PM -0500, Eric Sandeen wrote:
>>
>> Ok, fair enough.  If the BBU is working, nobarrier is ok; I don't trust
>> journal_async_commit, but that doesn't mean this isn't a regression.
> 
> Note that Toralf has reported almost exactly the same set of symptoms,
> but he's using an external USB stick --- and as far as I know he
> wasn't using nobarrier and/or the journal_async_commit.  Toralf, can
> you confirm what, if any, mount options you were using when you saw
> it.
> 
> I've been looking at this some more, and there's one other thing that
> the short circuit code does, which is neglects setting the
> JBD2_FLUSHED flag, which is used by the commit code to know when it
> needs to reset the s_start fields in the superblock when we make our
> next commit.  However, this would only happen if the short circuit
> code is getting hit some time other than when the file system is
> getting unmounted --- and that's what Eric and I can't figure out how
> it might be happening.  Journal flushes outside of an unmount does
> happen as part of online resizing, the FIBMAP ioctl, or when the file
> system is frozen.  But it didn't sound like Toralf or Nix was using
> any of those features.  (Toralf, Nix, please correct me if my
> assumptions here is wrong).

If I freeze w/ anything in the log, then s_start !=0 and we proceed
normally.  If I re-freeze w/o anything in the log, it's already set to
FLUSHED (which makes sense) so not re-setting it doesn't matter.  So I
don't see that that's an issue.

As for FIBMAP I think we only do journal_flush if it's data=journal.

In other news, Phoronix is on the case, so expect escalating freakouts ;)

-Eric