linux-kernel - Re: [PATCH] Allow userspace block device implementation

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Date:	Mon, 27 Jul 2009 09:46:57 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	axboe@...nel.dk, hch@...radead.org, akpm@...ux-foundation.org,
	Paul.Clements@...eleye.com, tytso@....edu,
	Tejun Heo <tj@...nel.org>, miklos <miklos@...redi.hu>
Subject: Re: [PATCH] Allow userspace block device implementation

Alan Cox wrote:
>> Somehow this made me think of FUSE/CUSE... should this be named aBUSE?
>> Oh wait it is :-), what I'm after is I guess is, can we share some of
>> the FUSE/CUSE code?

Well, it is A Block device in User SpacE :)  I don't think there is a
lot of code sharing benefit in some 800 odd lines, but I could be wrong.

> It reminds me of the existing and perfectly functional network block
> device (nbd) we already have and which has also been present for years.

Yes, I agree, in fact I looked at nbd as I was writing this, but I
believe it is different enough to warrant further investigation.

The network block device requires access to a socket, which the code at
least seems to imply brings up the potential for deadlocks when
self-hosting.  This was designed to explicitly support self-hosting.

This device can be used without CONFIG_NET (not a big advantage, I
agree), and is completely connectionless, which I would argue is a big
advantage.

NBD is perfectly functional, but it seemed more complicated than
necessary for a purely local implementation.  A fully functional null
server (just returns zeros, full error checking and normal whitespace)
can be implemented in about 60 lines of C code, which I don't think is
the case for NBD.  Of course, I'm sure it is possible with PERL bindings
as a one-liner, but the fundamental argument isn't about lines, it's
about complexity.  NBD requires socket allocation, listening and
connection; this requires only opening of a device node.

Can you swap over NBD?  Assuming one had pinned the userspace program
and it pre-allocated all memory so no pagein / alloc was required, would
it be deadlock proof?  I believe there are structure allocations
required for the socket implementation that go beyond the basic BIO
allocations, therefore making it impossible.  In /theory/, one should be
able to swap over this device.  In practice, it's probably a really bad
idea.

It seems then that NBD is a strict subset of the functionality provided
by this type of module.

Zach
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