Re: [PATCH v1 04/11] io_uring/kbuf: add mmap support for kernel-managed buffer rings

[Jens Axboe <axboe@kernel.dk>]

On 2/9/26 5:28 PM, Joanne Koong wrote:
> Add support for mmapping kernel-managed buffer rings (kmbuf) to
> userspace, allowing applications to access the kernel-allocated buffers.
> 
> Similar to application-provided buffer rings (pbuf), kmbuf rings use the
> buffer group ID encoded in the mmap offset to identify which buffer ring
> to map. The implementation follows the same pattern as pbuf rings.
> 
> New mmap offset constants are introduced:
>   - IORING_OFF_KMBUF_RING (0x88000000): Base offset for kmbuf mappings
>   - IORING_OFF_KMBUF_SHIFT (16): Shift value to encode buffer group ID
> 
> The mmap offset encodes the bgid shifted by IORING_OFF_KMBUF_SHIFT.
> The io_buf_get_region() helper retrieves the appropriate region.
> 
> This allows userspace to mmap the kernel-allocated buffer region and
> access the buffers directly.
> 
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---
>  include/uapi/linux/io_uring.h |  2 ++
>  io_uring/kbuf.c               | 11 +++++++++--
>  io_uring/kbuf.h               |  5 +++--
>  io_uring/memmap.c             |  5 ++++-
>  4 files changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> index a0889c1744bd..42a2812c9922 100644
> --- a/include/uapi/linux/io_uring.h
> +++ b/include/uapi/linux/io_uring.h
> @@ -545,6 +545,8 @@ struct io_uring_cqe {
>  #define IORING_OFF_SQES			0x10000000ULL
>  #define IORING_OFF_PBUF_RING		0x80000000ULL
>  #define IORING_OFF_PBUF_SHIFT		16
> +#define IORING_OFF_KMBUF_RING		0x88000000ULL
> +#define IORING_OFF_KMBUF_SHIFT		16
>  #define IORING_OFF_MMAP_MASK		0xf8000000ULL
>  
>  /*
> diff --git a/io_uring/kbuf.c b/io_uring/kbuf.c
> index 9bc36451d083..ccf5b213087b 100644
> --- a/io_uring/kbuf.c
> +++ b/io_uring/kbuf.c
> @@ -770,16 +770,23 @@ int io_register_pbuf_status(struct io_ring_ctx *ctx, void __user *arg)
>  	return 0;
>  }
>  
> -struct io_mapped_region *io_pbuf_get_region(struct io_ring_ctx *ctx,
> -					    unsigned int bgid)
> +struct io_mapped_region *io_buf_get_region(struct io_ring_ctx *ctx,
> +					   unsigned int bgid,
> +					   bool kernel_managed)
>  {
>  	struct io_buffer_list *bl;
> +	bool is_kernel_managed;
>  
>  	lockdep_assert_held(&ctx->mmap_lock);
>  
>  	bl = xa_load(&ctx->io_bl_xa, bgid);
>  	if (!bl || !(bl->flags & IOBL_BUF_RING))
>  		return NULL;
> +
> +	is_kernel_managed = !!(bl->flags & IOBL_KERNEL_MANAGED);
> +	if (is_kernel_managed != kernel_managed)
> +		return NULL;
> +
>  	return &bl->region;
>  }

For this, I think just add another helper - leave io_pbuf_get_region()
and add a bl->flags & IOBL_KERNEL_MANAGED error check in there, and
add a io_kbuf_get_region() or similar and have a !(bl->flags &
IOBL_KERNEL_MANAGED) error check in that one.

That's easier to read, and there's little reason to avoid duplicating
the xa_load() part.

Minor nit, but imho it's more readable that way.

-- 
Jens Axboe