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