[PATCHv9 5/7] io_uring: enable per-io hinting capability

[Anuj gupta]

On Sat, Oct 26, 2024 at 3:13 AM Keith Busch <kbusch at meta.com> wrote:
>
> From: Kanchan Joshi <joshi.k at samsung.com>
>
> With F_SET_RW_HINT fcntl, user can set a hint on the file inode, and
> all the subsequent writes on the file pass that hint value down. This
> can be limiting for block device as all the writes will be tagged with
> only one lifetime hint value. Concurrent writes (with different hint
> values) are hard to manage. Per-IO hinting solves that problem.
>
> Allow userspace to pass additional metadata in the SQE.
>
>         __u16 write_hint;
>
> If the hint is provided, filesystems may optionally use it. A filesytem
> may ignore this field if it does not support per-io hints, or if the
> value is invalid for its backing storage. Just like the inode hints,
> requesting values that are not supported by the hardware are not an
> error.
>
> Reviewed-by: Hannes Reinecke <hare at suse.de>
> Signed-off-by: Kanchan Joshi <joshi.k at samsung.com>
> Signed-off-by: Nitesh Shetty <nj.shetty at samsung.com>
> Signed-off-by: Keith Busch <kbusch at kernel.org>
> ---
>  include/uapi/linux/io_uring.h | 4 ++++
>  io_uring/rw.c                 | 3 ++-
>  2 files changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> index 60b9c98595faf..8cdcc461d464c 100644
> --- a/include/uapi/linux/io_uring.h
> +++ b/include/uapi/linux/io_uring.h
> @@ -92,6 +92,10 @@ struct io_uring_sqe {
>                         __u16   addr_len;
>                         __u16   __pad3[1];
>                 };
> +               struct {
> +                       __u16   write_hint;
> +                       __u16   __pad4[1];
> +               };
>         };
>         union {
>                 struct {
> diff --git a/io_uring/rw.c b/io_uring/rw.c
> index 8080ffd6d5712..5a1231bfecc3a 100644
> --- a/io_uring/rw.c
> +++ b/io_uring/rw.c
> @@ -279,7 +279,8 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe,
>                 rw->kiocb.ki_ioprio = get_current_ioprio();
>         }
>         rw->kiocb.dio_complete = NULL;
> -
> +       if (ddir == ITER_SOURCE)
> +               rw->kiocb.ki_write_hint = READ_ONCE(sqe->write_hint);
>         rw->addr = READ_ONCE(sqe->addr);
>         rw->len = READ_ONCE(sqe->len);
>         rw->flags = READ_ONCE(sqe->rw_flags);
> --
> 2.43.5
>

Since this patch adds a couple of new fields, it makes sense to add
BUILD_BUG_ON() checks in io_uring_init for these fields to assert the
layout of struct io_uring_sqe. And probably a zero check for pad4 in
io_prep_rw.
--
Anuj Gupta