Re: [PATCH v2 00/10] ublk: add shared memory zero-copy support

[Keith Busch <kbusch@kernel.org>]

On Tue, Mar 31, 2026 at 11:31:51PM +0800, Ming Lei wrote:
> Hello,
> 
> Add shared memory based zero-copy (UBLK_F_SHMEM_ZC) support for ublk.
> 
> The ublk server and its client share a memory region (e.g. memfd or
> hugetlbfs file) via MAP_SHARED mmap. The server registers this region
> with the kernel via UBLK_U_CMD_REG_BUF, which pins the pages and
> builds a PFN maple tree. When I/O arrives, the driver looks up bio
> pages in the maple tree - if they match registered buffer pages, the
> data is used directly without copying.

I feel if you have the ability to change an application to cooperatively
share memfd's with a ublk server, then why wouldn't you go one step
further: use vhost-user-blk and skip the kernel entirely?

The value for the existing ublk zero-copy setup was that original/legacy
applications didn't need to change. Of course it has the limitation of
what the ublk server can access, but if you're trying to get around that
with cooperative clients, then why not go all the way? What is ublk
providing here?