linux-kernel - Re: [PATCH 2/4] dm: implement REQ

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Date:	Fri, 27 Aug 2010 16:24:04 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	jaxboe@...ionio.com, k-ueda@...jp.nec.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, dm-devel@...hat.com
Subject: Re: [PATCH 2/4] dm: implement REQ_FLUSH/FUA support

On Fri, Aug 27 2010 at  1:10pm -0400,
Tejun Heo <tj@...nel.org> wrote:

> This patch converts dm to support REQ_FLUSH/FUA instead of now
> deprecated REQ_HARDBARRIER.

Thanks for your continued work on this!

> * As __blk_rq_prep_clone() copies REQ_FUA, just advertising FUA
>   support is enough to pass through REQ_FUA to targets.

You're doing blk_queue_flush(md->queue, REQ_FLUSH | REQ_FUA); in 2
places:
1) generic dm_init_md_queue -- used for bio-based and request-based
2) request-based specific dm_init_request_based_queue.

Interestingly, we never used the old blk_queue_ordered() method for
bio-based DM yet it is now using blk_queue_flush().

But how can we blindly assume/advertise REQ_FUA?

Should we be taking more care to check each block device that DM
consumes to see if FUA is supported and only then advertise REQ_FUA?
DM already does this for discard support (see:
dm_table_supports_discards).

> Lightly tested linear, stripe, raid1, snap and crypt targets.

I tested the bio-based code with the LVM2 test suite and all tests
passed.

> Please proceed with caution as I'm not familiar with the code base.

As I shared in an earlier (private) mail, I'm unfortunately having
problems with request-based DM (when all patches in this series are
applied).  I'll be working on that more.

BTW, we can eliminate the dm_rq_is_flush_request() wrapper right?  I
think hch mentioned this at some point in one of the various threads.

Mike
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