Re: [PATCH v4 06/25] io_uring/kbuf: add buffer ring pinning/unpinning

[Bernd Schubert <bernd@bsbernd.com>]

On 1/17/26 00:30, Joanne Koong wrote:
> Add kernel APIs to pin and unpin buffer rings, preventing userspace from
> unregistering a buffer ring while it is pinned by the kernel.
> 
> This provides a mechanism for kernel subsystems to safely access buffer
> ring contents while ensuring the buffer ring remains valid. A pinned
> buffer ring cannot be unregistered until explicitly unpinned. On the
> userspace side, trying to unregister a pinned buffer will return -EBUSY.
> 
> This is a preparatory change for upcoming fuse usage of kernel-managed
> buffer rings. It is necessary for fuse to pin the buffer ring because
> fuse may need to select a buffer in atomic contexts, which it can only
> do so by using the underlying buffer list pointer.
> 
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---
>  include/linux/io_uring/cmd.h | 17 +++++++++++++
>  io_uring/kbuf.c              | 48 ++++++++++++++++++++++++++++++++++++
>  io_uring/kbuf.h              |  5 ++++
>  3 files changed, 70 insertions(+)
> 
> diff --git a/include/linux/io_uring/cmd.h b/include/linux/io_uring/cmd.h
> index 375fd048c4cb..702b1903e6ee 100644
> --- a/include/linux/io_uring/cmd.h
> +++ b/include/linux/io_uring/cmd.h
> @@ -84,6 +84,10 @@ struct io_br_sel io_uring_cmd_buffer_select(struct io_uring_cmd *ioucmd,
>  bool io_uring_mshot_cmd_post_cqe(struct io_uring_cmd *ioucmd,
>  				 struct io_br_sel *sel, unsigned int issue_flags);
>  
> +int io_uring_buf_ring_pin(struct io_uring_cmd *cmd, unsigned buf_group,
> +			  unsigned issue_flags, struct io_buffer_list **bl);
> +int io_uring_buf_ring_unpin(struct io_uring_cmd *cmd, unsigned buf_group,
> +			    unsigned issue_flags);
>  #else
>  static inline int
>  io_uring_cmd_import_fixed(u64 ubuf, unsigned long len, int rw,
> @@ -126,6 +130,19 @@ static inline bool io_uring_mshot_cmd_post_cqe(struct io_uring_cmd *ioucmd,
>  {
>  	return true;
>  }
> +static inline int io_uring_buf_ring_pin(struct io_uring_cmd *cmd,
> +					unsigned buf_group,
> +					unsigned issue_flags,
> +					struct io_buffer_list **bl)
> +{
> +	return -EOPNOTSUPP;
> +}
> +static inline int io_uring_buf_ring_unpin(struct io_uring_cmd *cmd,
> +					  unsigned buf_group,
> +					  unsigned 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 d9bdb2be5f13..94ab23400721 100644
> --- a/io_uring/kbuf.c
> +++ b/io_uring/kbuf.c
> @@ -9,6 +9,7 @@
>  #include <linux/poll.h>
>  #include <linux/vmalloc.h>
>  #include <linux/io_uring.h>
> +#include <linux/io_uring/cmd.h>
>  
>  #include <uapi/linux/io_uring.h>
>  
> @@ -237,6 +238,51 @@ struct io_br_sel io_buffer_select(struct io_kiocb *req, size_t *len,
>  	return sel;
>  }
>  
> +int io_uring_buf_ring_pin(struct io_uring_cmd *cmd, unsigned buf_group,
> +			  unsigned issue_flags, struct io_buffer_list **bl)

I'm just looking at the fuse part and I'm actually not sure what the
"buf_group" parameter is. I guess it is a the buffer group set up by
userspace? Does io-uring have some documentation like fuse has under
Documentation/filesystems/fuse/?


Thanks,
Bernd