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

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Date:	Mon, 27 Jul 2009 22:02:18 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Zachary Amsden <zamsden@...hat.com>
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

> 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.

In practice since you mmap an object for write and the only free pages
left may be mmap write dirty pages to go to a file system it ought to be
possible.
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