SRU request for LP#208551

[Stefan Bader]

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Colin Ian King wrote:
> https://bugs.launchpad.net/ubuntu/hardy/+source/linux/+bug/208551
> 
> SRU justification:
> 
> Impact: mdadm, Raid5 get stuck in uninterruptable sleep under heavy I/O
> load. Copying data to a Raid 5 XFS partition results in a permanent lock
> on several processes related to it, getting stuck in the D(+) state.
> Occurs when large quantities of data (10-40 GB) is copied, resulting in
> processes being unkillable, and the system cannot reboot and requires
> power cycling the server.
> 
> Fix: The patch from commit 6ed3003c19a96fe18edf8179c4be6fe14abbebbc. The
> fix is to not make any generic_make_request() calls in raid5
> make_request until all waiting has been done.  We do this by simply
> setting STRIPE_HANDLE instead of calling handle_stripe(). This causes a
> performance hit, so this patch also only calls raid5_activate_delayed()
> at unplug time, never in raid5.  This seems to bring back the
> performance numbers. [quoting the commit message]
> 
> Testing: Without the patch, Raid 5 using md on an XFS filesystem locks
> up under heavy data copying - this is repeatable. With the patch, the
> lock up does not occur.
> 
> Patch tested in my PPA by Andrew Cholakian
> https://bugs.launchpad.net/ubuntu/hardy/+source/linux/+bug/208551/comments/16
> on 2 64 bit servers.
> 
> Patch attached.
> 
ACK. (Note: patch also in 2.6.26.y)


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