linux-kernel - Re: ext3fs: umount+sync not enough to guarantee metadata-on-disk

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Message-ID: <466A175C.1000906@rtr.ca>
Date:	Fri, 08 Jun 2007 22:58:36 -0400
From:	Mark Lord <lkml@....ca>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Chuck Ebbert <cebbert@...hat.com>,
	Stephen Tweedie <sct@...hat.com>,
	Theodore Ts'o <tytso@....edu>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: ext3fs: umount+sync not enough to guarantee metadata-on-disk

Andrew Morton wrote:
> On Thu, 07 Jun 2007 12:11:58 -0400
> Chuck Ebbert <cebbert@...hat.com> wrote:
> 
>> On 06/07/2007 11:41 AM, Andrew Morton wrote:
>>>>    mount /var/lib/mythtv -oremount,ro
>>>>    sync
>>>>    umount /var/lib/mythtv
>>> Did this succeed?  If the application is still truncating that file, the
>>> umount should have failed.
>> Shouldn't sync should wait for truncate to finish?
> 
> I can't think of anything in there at present which would cause that to
> happen, and it's not immediately obvious how we _could_ make it happen - we
> have an inode which potentially has no dirty pages and which is itself
> clean.  The truncate can span multiple journal commits, so forcing a
> journal commit in sync() won't necessarily block behind the truncate.
> 
> I guess we could ask sync to speculatively take and release every inode's
> i_mutex or something.  But even that would involve quite some hoop-jumping
> due to those infuriating spinlock-protected list_heads on the superblock.
> 
> hmm.

Okay, I added more instrumentation and retested today.

Good and Bad.
The umount does indeed fail while the massive unlink is happening,
so I can just loop on that a few times before giving up.

But.. the earlier "remount,ro".. well.. I don't know what it does.
I did get it to lock up solid, though.. hung on the "remount,ro"
when issued during an unlink of a 15GB file.  The disk I/O eventually
completes, and drives go idle, but the system remains hung inside
the remount,ro call.

Alt-sysrq-T was functioning, so I have some screen shots (.jpg) here:

  http://rtr.ca/remount_ro/

That's definitely a bug.
For now, I'll just not attempt the remount,ro on this system,
and have it loop for a minute attempting umount instead.

Cheers
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