linux-kernel - Re: [PATCH 5/5] dm: implement REQ

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Date:	Tue, 17 Aug 2010 14:21:31 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	jaxboe@...ionio.com, linux-fsdevel@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-ide@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
	hch@....de, James.Bottomley@...e.de, tytso@....edu,
	chris.mason@...cle.com, swhiteho@...hat.com,
	konishi.ryusuke@....ntt.co.jp, dm-devel@...hat.com, vst@...b.net,
	jack@...e.cz, rwheeler@...hat.com, hare@...e.de, neilb@...e.de,
	rusty@...tcorp.com.au, mst@...hat.com,
	Mikulas Patocka <mpatocka@...hat.com>,
	Kiyoshi Ueda <k-ueda@...jp.nec.com>,
	"Jun'ichi Nomura" <j-nomura@...jp.nec.com>
Subject: Re: [PATCH 5/5] dm: implement REQ_FLUSH/FUA support

On Tue, Aug 17 2010 at 12:51pm -0400,
Tejun Heo <tj@...nel.org> wrote:

> Hello,
> 
> On 08/17/2010 04:07 PM, Mike Snitzer wrote:
> >> With the patch applied, there's no second flush.  Those requests would
> >> now be REQ_FLUSH + REQ_DISCARD.  The first can't be avoided anyway and
> >> there won't be the second flush to begin with, so I don't think this
> >> worsens anything.
> > 
> > Makes sense, but your patches still need to be refreshed against the
> > latest (2.6.36-rc1) upstream code.  Numerous changes went in to DM
> > recently.
> 
> Sure thing.  The block part isn't fixed yet and so the RFC tag.  Once
> the block layer part is settled, it probably should be pulled into
> dm/md and other trees and conversions should happen there.

Why base your work on a partial 2.6.36 tree?  Why not rebase to linus'
2.6.36-rc1?

Once we get the changes in a more suitable state (across the entire
tree) we can split the individual changes out to their respective
trees.  Without a comprehensive tree I fear this code isn't going to get
tested or reviewed properly.

For example: any review of DM changes, against stale DM code, is wasted
effort.

> >> * For request based dm:
> >>
> >>   * The sequencing is done by the block layer for the top level
> >>     request_queue, so the only things request based dm needs to make
> >>     sure is 1. handling empty REQ_FLUSH correctly (block layer will
> >>     only send down empty REQ_FLUSHes) and 2. propagate REQ_FUA bit to
> >>     member devices.
> > 
> > OK, so seems 1 is done, 2 is still TODO.  Looking at your tree it seems
> > 2 would be as simple as using the following in
> 
> Oh, I was talking about the other way around.  Passing REQ_FUA in
> bio->bi_rw down to member request_queues.  Sometimes while
> constructing clone / split bios, the bit is lost (e.g. md raid5).

Seems we need to change __blk_rq_prep_clone to propagate REQ_FUA just
like REQ_DISCARD: http://git.kernel.org/linus/3383977

Doesn't seem like we need to do the same for REQ_FLUSH (at least not for
rq-based DM's benefit) because dm.c:setup_clone already special cases
flush requests and sets REQ_FLUSH in cmd_flags.

Mike
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