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
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