[PATCH v1 0/5] dm: dm-user: New target that proxies BIOs to userspace

[Palmer Dabbelt <palmer@dabbelt.com>]

This patch set contains dm-user, a device mapper target that proxies incoming
BIOs to userspace via a misc device.  Essentially it's FUSE, but for block
devices.  There's more information in the documentation patch and as a handful
of commends, so I'm just going to avoid duplicating that here.  I don't really
think there's any fundamental functionality that dm-user enables, as one could
use something along the lines of nbd/iscsi, but dm-user does result in
extremely simple userspace daemons -- so simple that when I tried to write a
helper userspace library for dm-user I just ended up with nothing.

I talked about this a bit at Plumbers and was hoping to send patches a bit
earlier on in the process, but got tied up with a few things.  As a result this
is actually quite far along: it's at the point where we're starting to run this
on real devices as part of an updated Android OTA update flow, where we're
using this to provide an Android-specific compressed backing store for
dm-snap-persistent.  The bulk of that project is scattered throughout the
various Android trees, so there are kselftests and a (somewhat bare for now)
Documentation entry with the intent of making this a self-contained
contribution.  There's a lot to the Android userspace daemon, but it doesn't
interact with dm-user in a very complex manner.

This is still in a somewhat early stage, but it's at the point where things
largely function.  I'm certainly not ready to commit to the user ABI
implemented here and there are a bunch of FIXMEs scattered throughout the code,
but I do think that it's far along enough to begin a more concrete discussion
of where folks would like to go with something like this.  While I'd intending
on sorting that stuff out, I'd like to at least get a feel for whether this is
a path worth pursuing before spending a bunch more time on it.

I haven't done much in the way of performance analysis for dm-user.  Earlier on
I did some simple throughput tests and found that dm-user/ext4 was faster than
half the speed of tmpfs, which is way out of the realm of being an issue for
our use case (decompressing blocks out of a phone's storage).  The design of
dm-user does preclude an extremely high performance implementation, where I
assume one would want an explicit ring buffer and zero copy, but I feel like
users who want that degree of performance are probably better served writing a
proper kernel driver.  I wouldn't be opposed to pushing on performance (ideally
without a major design change), but for now I feel like time is better spent
fortifying the user ABI and fixing the various issues with the implementation.

The patches follow as usual, but in case it's easier I've published a tree as
well:

    git://git.kernel.org/pub/scm/linux/kernel/git/palmer/dm-user.git -b dm-user-v1