Re: [PATCH v3 07/25] io_uring/kbuf: add recycling for kernel managed buffer rings

[Caleb Sander Mateos <csander@purestorage.com>]

On Mon, Dec 22, 2025 at 4:36 PM Joanne Koong <joannelkoong@gmail.com> wrote:
>
> Add an interface for buffers to be recycled back into a kernel-managed
> buffer ring.
>
> This is a preparatory patch for fuse over io-uring.
>
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---
>  include/linux/io_uring/cmd.h | 13 +++++++++++
>  io_uring/kbuf.c              | 42 ++++++++++++++++++++++++++++++++++++
>  io_uring/kbuf.h              |  3 +++
>  io_uring/uring_cmd.c         | 11 ++++++++++
>  4 files changed, 69 insertions(+)
>
> diff --git a/include/linux/io_uring/cmd.h b/include/linux/io_uring/cmd.h
> index 424f071f42e5..7169a2a9a744 100644
> --- a/include/linux/io_uring/cmd.h
> +++ b/include/linux/io_uring/cmd.h
> @@ -88,6 +88,11 @@ int io_uring_cmd_buf_ring_pin(struct io_uring_cmd *ioucmd, unsigned buf_group,
>                               unsigned issue_flags, struct io_buffer_list **bl);
>  int io_uring_cmd_buf_ring_unpin(struct io_uring_cmd *ioucmd, unsigned buf_group,
>                                 unsigned issue_flags);
> +
> +int io_uring_cmd_kmbuffer_recycle(struct io_uring_cmd *cmd,
> +                                 unsigned int buf_group, u64 addr,
> +                                 unsigned int len, unsigned int bid,
> +                                 unsigned int issue_flags);
>  #else
>  static inline int
>  io_uring_cmd_import_fixed(u64 ubuf, unsigned long len, int rw,
> @@ -143,6 +148,14 @@ static inline int io_uring_cmd_buf_ring_unpin(struct io_uring_cmd *ioucmd,
>  {
>         return -EOPNOTSUPP;
>  }
> +static inline int io_uring_cmd_kmbuffer_recycle(struct io_uring_cmd *cmd,
> +                                               unsigned int buf_group,
> +                                               u64 addr, unsigned int len,
> +                                               unsigned int bid,
> +                                               unsigned int issue_flags)
> +{
> +       return -EOPNOTSUPP;
> +}
>  #endif
>
>  static inline struct io_uring_cmd *io_uring_cmd_from_tw(struct io_tw_req tw_req)
> diff --git a/io_uring/kbuf.c b/io_uring/kbuf.c
> index 03e05bab023a..f12d000b71c5 100644
> --- a/io_uring/kbuf.c
> +++ b/io_uring/kbuf.c
> @@ -101,6 +101,48 @@ void io_kbuf_drop_legacy(struct io_kiocb *req)
>         req->kbuf = NULL;
>  }
>
> +int io_kmbuf_recycle(struct io_kiocb *req, unsigned int bgid, u64 addr,
> +                    unsigned int len, unsigned int bid,
> +                    unsigned int issue_flags)
> +{
> +       struct io_ring_ctx *ctx = req->ctx;
> +       struct io_uring_buf_ring *br;
> +       struct io_uring_buf *buf;
> +       struct io_buffer_list *bl;
> +       int ret = -EINVAL;
> +
> +       if (WARN_ON_ONCE(req->flags & REQ_F_BUFFERS_COMMIT))
> +               return ret;
> +
> +       io_ring_submit_lock(ctx, issue_flags);
> +
> +       bl = io_buffer_get_list(ctx, bgid);
> +
> +       if (WARN_ON_ONCE(!(bl->flags & IOBL_BUF_RING)) ||
> +           WARN_ON_ONCE(!(bl->flags & IOBL_KERNEL_MANAGED)))
> +               goto done;
> +
> +       br = bl->buf_ring;
> +
> +       if (WARN_ON_ONCE((br->tail - bl->head) >= bl->nr_entries))
> +               goto done;
> +
> +       buf = &br->bufs[(br->tail) & bl->mask];
> +
> +       buf->addr = addr;
> +       buf->len = len;
> +       buf->bid = bid;
> +
> +       req->flags &= ~REQ_F_BUFFER_RING;
> +
> +       br->tail++;
> +       ret = 0;
> +
> +done:
> +       io_ring_submit_unlock(ctx, issue_flags);
> +       return ret;
> +}
> +
>  bool io_kbuf_recycle_legacy(struct io_kiocb *req, unsigned issue_flags)
>  {
>         struct io_ring_ctx *ctx = req->ctx;
> diff --git a/io_uring/kbuf.h b/io_uring/kbuf.h
> index c4368f35cf11..4d8b7491628e 100644
> --- a/io_uring/kbuf.h
> +++ b/io_uring/kbuf.h
> @@ -146,4 +146,7 @@ int io_kbuf_ring_pin(struct io_kiocb *req, unsigned buf_group,
>                      unsigned issue_flags, struct io_buffer_list **bl);
>  int io_kbuf_ring_unpin(struct io_kiocb *req, unsigned buf_group,
>                        unsigned issue_flags);
> +int io_kmbuf_recycle(struct io_kiocb *req, unsigned int bgid, u64 addr,
> +                    unsigned int len, unsigned int bid,
> +                    unsigned int issue_flags);
>  #endif
> diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
> index 8ac79ead4158..b6b675010bfd 100644
> --- a/io_uring/uring_cmd.c
> +++ b/io_uring/uring_cmd.c
> @@ -416,3 +416,14 @@ int io_uring_cmd_buf_ring_unpin(struct io_uring_cmd *ioucmd, unsigned buf_group,
>         return io_kbuf_ring_unpin(req, buf_group, issue_flags);
>  }
>  EXPORT_SYMBOL_GPL(io_uring_cmd_buf_ring_unpin);
> +
> +int io_uring_cmd_kmbuffer_recycle(struct io_uring_cmd *ioucmd,
> +                                 unsigned int buf_group, u64 addr,
> +                                 unsigned int len, unsigned int bid,
> +                                 unsigned int issue_flags)
> +{
> +       struct io_kiocb *req = cmd_to_io_kiocb(ioucmd);
> +
> +       return io_kmbuf_recycle(req, buf_group, addr, len, bid, issue_flags);

Total nit, but is there a reason for the inconsistency between
"kmbuffer" and "kmbuf"? I would prefer to use a single name for the
concept.

Best,
Caleb

> +}
> +EXPORT_SYMBOL_GPL(io_uring_cmd_kmbuffer_recycle);
> --
> 2.47.3
>