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

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

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...


thanks,
Ming