Re: [LSF/MM/BPF TOPIC] block drivers in user space

[Hannes Reinecke <hare@suse.de>]

On 2/21/22 20:59, Gabriel Krisman Bertazi wrote:
> I'd like to discuss an interface to implement user space block devices,
> while avoiding local network NBD solutions.  There has been reiterated
> interest in the topic, both from researchers [1] and from the community,
> including a proposed session in LSFMM2018 [2] (though I don't think it
> happened).
> 
> I've been working on top of the Google iblock implementation to find
> something upstreamable and would like to present my design and gather
> feedback on some points, in particular zero-copy and overall user space
> interface.
> 
> The design I'm pending towards uses special fds opened by the driver to
> transfer data to/from the block driver, preferably through direct
> splicing as much as possible, to keep data only in kernel space.  This
> is because, in my use case, the driver usually only manipulates
> metadata, while data is forwarded directly through the network, or
> similar. It would be neat if we can leverage the existing
> splice/copy_file_range syscalls such that we don't ever need to bring
> disk data to user space, if we can avoid it.  I've also experimented
> with regular pipes, But I found no way around keeping a lot of pipes
> opened, one for each possible command 'slot'.
> 
> [1] https://dl.acm.org/doi/10.1145/3456727.3463768
> [2] https://www.spinics.net/lists/linux-fsdevel/msg120674.html
> 
Actually, I'd rather have something like an 'inverse io_uring', where an 
application creates a memory region separated into several 'ring' for 
submission and completion.
Then the kernel could write/map the incoming data onto the rings, and 
application can read from there.
Maybe it'll be worthwhile to look at virtio here.

But in either case, using fds or pipes for commands doesn't really 
scale, as the number of fds is inherently limited. And using fds 
restricts you to serial processing (as you can read only sequentially 
from a fd); with mmap() you'll get a greater flexibility and the option 
of parallel processing.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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