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Date:	Sat, 8 Mar 2014 12:33:20 -0500
From:	Mike Snitzer <snitzer@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Jeff Moyer <jmoyer@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	Shaohua Li <shli@...ionio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	msnitzer <msnitzer@...hat.com>, Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCH 1/1] block: rework flush sequencing for blk-mq

On Sat, Mar 8, 2014 at 10:52 AM, Christoph Hellwig <hch@...radead.org> wrote:
> On Fri, Mar 07, 2014 at 03:45:09PM -0500, Jeff Moyer wrote:
>> Hi, Christoph,
>>
>> Did you mean to switch from list_add to list_add_tail?  That seems like
>> a change that warrants mention.
>
> No, that wasn't intentional and should be fixed.  Btw, there was another
> issue with that commit, in that dm-multipath also needs to allocate
> ->flush_rq.  I saw a patch from Hannes fixing it in the SuSE tree, and
> would really love to see him submit that for mainline as well.

Ugh, rq-based DM calls blk_init_allocated_queue.. (ah, looks like best
to move q->flush_rq allocation from blk_init_queue_node to
blk_alloc_queue_node?).  Anyway, this all makes sense given the
crashes we've been dealing with.. we couldn't immediately understand
how the q->flush_rq would be NULL... grr.  I guess I should've checked
with you sooner.  I reverted commit 1874198 "blk-mq: rework flush
sequencing logic" from RHEL7 just yesterday because we were seeing
crashes on flush with dm-mpath.  But can easily re-apply for RHEL7.1
(since the request_queue's embedded flush_rq takes up so much space we
get ample kABI padding).

Not overly proud of the revert, but I deemed easier to revert than
hunt down the fix given RHEL7 won't actually be providing any blk-mq
enabled drivers.  That'll change for RHEL7.1.

> Unfortunately SuSE seems to have lots of block and dm fixes and even
> features that they don't submit upstream.

Yeah, it is certainly disturbing.  No excuse for sitting on fixes like this.

Hannes, _please_ get this dm-mpath flush_rq fix for 3.14 posted ASAP.
Jens or I will need to get it to Linus next week.

Thanks,
Mike
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