From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mike Snitzer <snitzer@redhat.com>
Subject: Re: blk-mq request allocation stalls
Date: Mon, 12 Jan 2015 15:21:14 -0500
Message-ID: <20150112202113.GA23234@redhat.com>
References: <54B3EB4A.9090404@kernel.dk> <54B3F255.5080802@sandisk.com>
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	<20150112191138.GC21518@redhat.com>
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To: Keith Busch <keith.busch@intel.com>
Cc: Jens Axboe <axboe@kernel.dk>, Bart Van Assche <bart.vanassche@sandisk.com>, device-mapper development <dm-devel@redhat.com>, Jun'ichi Nomura <j-nomura@ce.jp.nec.com>, Christoph Hellwig <hch@infradead.org>
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On Mon, Jan 12 2015 at  2:11pm -0500,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Mon, Jan 12 2015 at  1:35pm -0500,
> Keith Busch <keith.busch@intel.com> wrote:
> 
> > On Mon, 12 Jan 2015, Keith Busch wrote:
> > >Oh, let's look at "__blk_rq_prep_clone". dm calls that after
> > >blk_get_request() for the blk-mq based multipath types and overrides the
> > >destinations cmd_flags with the source's even though the source was not
> > >allocated from a blk-mq based queue, much less a shared tag.
> > 
> > Untested patch. This will also preserve the failfast cmd_flag dm-mpath
> > set after allocating.
> 
> Ah, good point.  The failfast flag would get cleared with the patch I
> just proposed (unless REQ_FAILFAST_TRANSPORT was added to
> REQ_PRESERVE_CLONE_MASK).
> 
> Anyway, I'm happy to see this implemented however you guys think is
> best.  I think I like Keith's patch better than mine.

FYI, I staged Keith's patch here:
https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-for-3.20-blk-mq&id=7004ddf2462df38c6e3232ac020ed6ff655cc07e

Bart, this is the tip of the linux-dm.git "dm-for-3.20-blk-mq" branch.
Please test, it should hopefully take care of the stall you've been
seeing.

Mike