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

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Message-ID: <CAH+dOxKr=BcWO5qmYZkrc2px+ckx=_+viWQ9M2hE7RMA3DaXNg@mail.gmail.com>
Date:	Wed, 8 Aug 2012 20:25:03 -0700
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Muthu Kumar <muthu.lkml@...il.com>
Cc:	Tejun Heo <tj@...nel.org>, 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, 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

On Wed, Aug 8, 2012 at 8:19 PM, Kent Overstreet <koverstreet@...gle.com> wrote:
> In particular, if this change breaks anything then the new bio_split()
> _will_ break things.
>
> We need to be clear about our interfaces; in this case bi_idx and
> bi_vcnt, in particular. Either this is a safe change, or it's not. If
> no one knows... that's a bigger problem, and not just for this patch...
>
> Fortunately this code actually has been tested quite a bit (and the bio
> splitting code for even longer), and (somewhat to my surprise) I haven't
> run into any bugs caused by it.

Oh, it's worse than that - remember how bio_kmalloc() works for up to
1024 pages, but bio_alloc_bioset() only up to 256?

We can already have situations where bios are allocated that are
impossible to clone (or if we don't, it's only because of
queue_limits. That's not sketchy at all.)
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