linux-ext4 - kernel thread "flush-254:12" eats 100% CPU

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Date:	Wed, 07 Sep 2011 17:46:37 +0200
From:	Marcin M <gmane@...or.pl>
To:	linux-ext4@...r.kernel.org
Subject: kernel thread "flush-254:12" eats 100% CPU

Hello!
I hope i'm writing to correct place (but i suspect g.l.kernel list could 
be proper also) :)
I'm observing situation like in subject. From time to time 
"flush-254:12" takes 100% of cpu for a couple of minutes. Then 
everything becomes ok.
This problem appears on my two different boxes.

box A)
bare metal, i686, hardened-kernel-2.6.{37-38}
on this box kernel thread never stops to do 100% of cpu. I've to reboot 
box (using sysrq, because i could't even umount partition associated 
with this thread)
My workarround for it: i changed ext4 to xfs on dm device and problem 
disappeared

box B)
xen, full virtualization, x86_64, kernels: 
hardened-kernel-{2.6.39,3.0.3,3.0.4} and 3.1.0-rc4-git2
problem is as i described above, flush took one cpu for a couple minutes 
and then everythings works correctly.

And again, 254-12 device is ext4 filesystem.
In both cases it was used lvm. At box B, additionaly i'm using dmcrypt. 
Box B is configured in this way: sda->lvm2->dmcrypt->filesystem.

How can i help to debug this problem? Or maybe it's fixed already?

Regards,
Marcin.

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