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

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Date:	Wed, 6 Nov 2013 15:36:40 -0500
From:	Mike Snitzer <snitzer@...hat.com>
To:	Chris Mason <chris.mason@...ionio.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

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?

Mike
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