linux-kernel - Re: [PATCH UPDATED 4/5] dm: implement REQ_FLUSH/FUA support for request-based dm

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Date:	Wed, 1 Sep 2010 08:25:27 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Kiyoshi Ueda <k-ueda@...jp.nec.com>
Cc:	Tejun Heo <tj@...nel.org>, jaxboe@...ionio.com,
	j-nomura@...jp.nec.com, jamie@...reable.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-raid@...r.kernel.org, hch@....de
Subject: Re: [PATCH UPDATED 4/5] dm: implement REQ_FLUSH/FUA support for
 request-based dm

On Wed, Sep 01 2010 at  3:15am -0400,
Kiyoshi Ueda <k-ueda@...jp.nec.com> wrote:

> Hi Tejun,
> 
> On 08/31/2010 12:45 AM +0900, Tejun Heo wrote:
> > This patch converts request-based dm to support the new REQ_FLUSH/FUA.
> > 
> > The original request-based flush implementation depended on
> > request_queue blocking other requests while a barrier sequence is in
> > progress, which is no longer true for the new REQ_FLUSH/FUA.
> > 
> > In general, request-based dm doesn't have infrastructure for cloning
> > one source request to multiple targets, but the original flush
> > implementation had a special mostly independent path which can issue
> > flushes to multiple targets and sequence them.  However, the
> > capability isn't currently in use and adds a lot of complexity.
> > Moreoever, it's unlikely to be useful in its current form as it
> > doesn't make sense to be able to send out flushes to multiple targets
> > when write requests can't be.
> > 
> > This patch rips out special flush code path and deals handles
> > REQ_FLUSH/FUA requests the same way as other requests.  The only
> > special treatment is that REQ_FLUSH requests use the block address 0
> > when finding target, which is enough for now.
> > 
> > * added BUG_ON(!dm_target_is_valid(ti)) in dm_request_fn() as
> >   suggested by Mike Snitzer
> 
> Thank you for your work.
> 
> I don't see any obvious problem on this patch.
> However, I hit a NULL pointer dereference below when I use a mpath
> device with barrier option of ext3.  I'm investigating the cause now.
> (Also I'm not sure the cause of the hang which Mike is hitting yet.)
> 
> I tried on the commit 28dd53b26d362c16234249bad61db8cbd9222d0b of
> git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git flush-fua.
> 
>     # mke2fs -j /dev/mapper/mpatha
>     # mount -o barrier=1 /dev/mapper/mpatha /mnt/0
>     # dd if=/dev/zero of=/mnt/0/a bs=512 count=1
>     # sync
> 
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000078

FYI, I can't reproduce this using all of Tejun's latest patches (not yet
in the flush-fua git tree).  But I haven't tried the specific flush-fua
commit that you referenced.

Mike
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