From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mike Snitzer <snitzer@redhat.com>
Subject: Re: [PATCH v3 0/8] dm: add request-based blk-mq support
Date: Wed, 24 Dec 2014 13:55:29 -0500
Message-ID: <20141224185529.GA13246@redhat.com>
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Cc: Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@infradead.org>, Keith Busch <keith.busch@intel.com>, device-mapper development <dm-devel@redhat.com>, Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
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On Wed, Dec 24 2014 at  1:21pm -0500,
Mike Snitzer <snitzer@redhat.com> wrote:

> This is odd considering blk-mq defaults to setting QUEUE_FLAG_IO_STAT,
> so each request will have REQ_IO_STAT set.
> 
> I'm not sure what would account for this NULL pointer (my code appears
> to be slightly different than yours but AFAICT req->part is NULL in your
> crash, which shouldn't ever happen if IO stats are enabled).
> 
> Are you manually enabling/disabling IO stats via sysfs at all?

Answering my own question: unlikely.  Considering I was able to
reproduce merely by creating an mpath device ontop of a blk-mq device
(now that dm-mpath.c:__multipath_map sets rq->rq_disk).

I'll try to get to the bottom of it (it still seems pretty weird to me).