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

[Ming Lei <ming.lei@redhat.com>]

On Tue, Apr 07, 2026 at 12:29:39PM -0700, Caleb Sander Mateos wrote:
> On Mon, Apr 6, 2026 at 7:39 PM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > 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.
> > >
> > > Please see details on document added in patch 3.
> > >
> > > Patches 1-4 implement the kernel side:
> > >  - buffer register/unregister control commands with PFN coalescing,
> > >    including read-only buffer support (UBLK_SHMEM_BUF_READ_ONLY)
> > >  - PFN-based matching in the I/O path, with enforcement that read-only
> > >    buffers reject non-WRITE requests
> > >  - UBLK_F_SHMEM_ZC feature flag
> > >  - eliminate permanent pages[] array from struct ublk_buf; the maple
> > >    tree already stores PFN ranges, so pages[] becomes temporary
> > >
> > > Patches 5-10 add kublk (selftest server) support and tests:
> > >  - hugetlbfs buffer sharing (both kublk and fio mmap the same file)
> > >  - null target and loop target tests with fio verify
> > >  - filesystem-level test (ext4 on ublk, fio verify on a file)
> > >  - read-only buffer registration test (--rdonly_shmem_buf)
> > >
> > > Changes since V1:
> > >  - rename struct ublk_buf_reg to struct ublk_shmem_buf_reg, add __u32
> > >    flags field for extensibility, narrow __u64 len to __u32 (max 4GB
> > >    per UBLK_SHMEM_ZC_OFF_MASK), remove __u32 reserved (patch 1)
> > >  - add UBLK_SHMEM_BUF_READ_ONLY flag: pin pages without FOLL_WRITE,
> > >    enabling registration of write-sealed memfd buffers (patch 1)
> > >  - use backward-compatible struct reading: memset zero + copy
> > >    min(header->len, sizeof(struct)) (patch 1)
> > >  - reorder struct ublk_buf_range fields for better packing (16 bytes
> > >    vs 24 bytes), change buf_index to unsigned short, add unsigned short
> > >    flags to store per-range read-only state (patch 1)
> > >  - enforce read-only buffer semantics in ublk_try_buf_match(): reject
> > >    non-WRITE requests on read-only buffers since READ I/O needs to
> > >    write data into the buffer (patch 2)
> > >  - narrow struct ublk_buf::nr_pages to unsigned int, narrow struct
> > >    ublk_buf_range::base_offset to unsigned int (patch 1)
> > >  - add new patch 4: eliminate permanent pages[] array from struct
> > >    ublk_buf — recover struct page pointers via pfn_to_page() from the
> > >    maple tree during unregistration, saving 2MB per 1GB buffer
> > >  - add UBLK_F_SHMEM_ZC to feat_map in kublk (patch 5)
> > >  - add new patch 10: read-only buffer registration selftest with
> > >    --rdonly_shmem_buf option on null target + hugetlbfs
> >
> > Hello,
> >
> > Ping...
> 
> Sorry I didn't get a chance to look at this earlier. It looks really
> nice, thank you for implementing it! I have just a few comments. One
> is about the UAPI (can we just use virtual addresses instead of buffer
> index + offset), so I might wait on landing this patchset until we've
> finalized that.

Hi Jens,

Can you drop V2 from for-7.1/block so that we can polish up it in V3?

Or I am fine to cook up a followup for misc clean & fix?

Thanks,
Ming