linux-kernel - Re: [dm-devel] [PATCH v5 12/12] block: Only clone bio vecs that are in use

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Message-ID: <20120809070154.GG2845@dhcp-172-17-108-109.mtv.corp.google.com>
Date:	Thu, 9 Aug 2012 00:01:54 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Muthu Kumar <muthu.lkml@...il.com>
Cc:	Mikulas Patocka <mpatocka@...hat.com>,
	device-mapper development <dm-devel@...hat.com>,
	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	axboe@...nel.dk, Kent Overstreet <koverstreet@...gle.com>,
	vgoyal@...hat.com, yehuda@...newdream.net, sage@...dream.net,
	agk@...hat.com, drbd-dev@...ts.linbit.com
Subject: Re: [dm-devel] [PATCH v5 12/12] block: Only clone bio vecs that
 are in use

Hello,

On Wed, Aug 08, 2012 at 04:47:46PM -0700, Muthu Kumar wrote:
> You are changing the meaning of __bio_clone() here. In old code, the
> number of io_vecs, bi_idx, bi_vcnt are preserved. But in this modified
> code, you are mapping bio_src's bi_iovec[bi_idx] to bio_dests
> bi_iovec[0] and also restricting the number of allocated io_vecs of
> the clone. It may be useful for cases were we would like a identical
> copy of the original bio (may not be in current code base, but this
> implementation is definitely not what one would expect from the name
> "clone").

Implementation details changed somewhat but the high-level semantics
didn't change at all.  Any driver not messing with bio internals - and
they shouldn't - shouldn't notice the change.  No in-kernel drivers
seem to be broken by the change.  If you ask me, this looks more like
a bug fix to me where the bug is a silly behavior restricting
usefulness of the interface.

> May be, call this new implementation some thing else (and use it for bcache)?

This doesn't only change __bio_clone() but all clone interface stacked
on top of it, so, no way.  This ain't windows.

Thanks.

-- 
tejun
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