linux-kernel - Re: [PATCH v6 11/13] block: Rework bio_pair

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Date:	Fri, 24 Aug 2012 03:37:34 -0700
From:	Kent Overstreet <koverstreet@...gle.com>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
Cc:	Tejun Heo <tj@...nel.org>, linux-bcache@...r.kernel.org,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com,
	vgoyal@...hat.com, mpatocka@...hat.com, bharrosh@...asas.com,
	Jens Axboe <axboe@...nel.dk>, NeilBrown <neilb@...e.de>,
	Lars Ellenberg <lars.ellenberg@...bit.com>,
	Peter Osterlund <petero2@...ia.com>,
	Sage Weil <sage@...tank.com>
Subject: Re: [PATCH v6 11/13] block: Rework bio_pair_split()

On Thu, Aug 23, 2012 at 10:25:47PM -0400, Martin K. Petersen wrote:
> >>>>> "Tejun" == Tejun Heo <tj@...nel.org> writes:
> 
> Tejun> I complained about this in the last posting and in the previous
> Tejun> patch.  Please respond.  Martin, are you okay with these
> Tejun> integrity changes?
> 
> I missed the first several iterations of all this while I was out on
> vacation. I'll have to try to wrap my head around the new approach.
> 
> However, I'm not sure I like the overall approach of the new splitting.
> Instead of all this cloning, slicing and dicing of bio_vecs I'd rather
> we bit the bullet and had an offset + length for the vector inside each
> bio. That way we could keep the bio_vec immutable and make clones more
> lightweight since their vecs would always point to the parent. This also
> makes it trivial to split I/Os in the stacking drivers and removes evils
> in the partial completion code path. It would also allow to sever the
> ties between "size of block range operated on" vs. bi_size which we need
> for copy offload, discard, etc.

Agree 110% - making bio_vecs immutable and keeping the offset in the bio
is something I've been talking about for ages, I'd love to see it
happen.

But that's going to be a much more invasive change so if I'm going to do
it (and I am willing to work on it) it's just going to be a bit. This is
really a stopgap solution.

As far as the integrity splitting, it's similar to what the existing dm
code does (main difference is dm already has the bio cloned, my
bio_split() doesn't assume anything about the bio being split). Not sure
how that affects ownership of the integrity data, honestly that part
kind of confuses me.
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