linux-kernel - Re: block: Revert bio_clone() default behaviour

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Message-ID: <20131106204952.3802.28112@localhost.localdomain>
Date:	Wed, 6 Nov 2013 15:49:52 -0500
From:	Chris Mason <chris.mason@...ionio.com>
To:	Mike Snitzer <snitzer@...hat.com>
CC:	Kent Overstreet <kmo@...erainc.com>, <axboe@...nel.dk>,
	<linux-kernel@...r.kernel.org>, NeilBrown <neilb@...e.de>,
	Olof Johansson <olof@...om.net>
Subject: Re: block: Revert bio_clone() default behaviour

Quoting Mike Snitzer (2013-11-06 15:36:40)
> On Wed, Nov 06 2013 at  3:22pm -0500,
> Chris Mason <chris.mason@...ionio.com> wrote:
> 
> > Quoting Kent Overstreet (2013-11-06 15:02:22)
> > > On Wed, Nov 06, 2013 at 11:11:30AM -0500, Chris Mason wrote:
> > > > 
> > > > I think the concept of bio ownership is still much too weak, at least
> > > > for established users like MD and DM.  I don't know how to verify the
> > > > sharing of bi_io_vec without some kind of reference counting on the
> > > > iovec.
> > > 
> > > What's unclear about it? The rule is just - if you didn't allocate the biovec,
> > > don't modify it or use bio_for_each_segment_all() (probably I didn't quite state
> > > it clearly enough before though)
> > 
> > That part makes sense.  The new rule that scares me is that we can't
> > free the src of the clone until all the clones are freed.  If it works
> > with today's existing users it feels like it is more by accident than
> > design.  I'm not saying we can't do it, we just need some bigger
> > flashing warning lights.
> 
> But we probably don't want those warning lights to come with the cost of
> managing extra refcounts in the fast path -- so maybe a debug-only
> refcount?

I'd be happy with some code comments and a few extra SOBs.  In general
I'm happy with the new patches that add _fast and make us explicitly
choose the sharing.  Lots of little chances for bugs, but opt-in is a
much better starting point.

-chris
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