Date: Mon, 27 Jul 2009 22:02:18 +0100
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Zachary Amsden <zamsden@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
       torvalds@linux-foundation.org, axboe@kernel.dk, hch@infradead.org,
       akpm@linux-foundation.org, Paul.Clements@steeleye.com, tytso@mit.edu,
       Tejun Heo <tj@kernel.org>, miklos <miklos@szeredi.hu>
Subject: Re: [PATCH] Allow userspace block device implementation
Message-ID: <20090727220218.65a2adb2@lxorguk.ukuu.org.uk>
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> 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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