Re: [PATCH v3 06/25] io_uring/kbuf: add buffer ring pinning/unpinning

[Joanne Koong <joannelkoong@gmail.com>]

On Thu, Jan 8, 2026 at 11:18 AM Caleb Sander Mateos
<csander@purestorage.com> wrote:
>
> On Mon, Dec 22, 2025 at 4:36 PM Joanne Koong <joannelkoong@gmail.com> 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              | 46 ++++++++++++++++++++++++++++++++++++
> >  io_uring/kbuf.h              | 10 ++++++++
> >  io_uring/uring_cmd.c         | 18 ++++++++++++++
> >  4 files changed, 91 insertions(+)
> >
> > diff --git a/include/linux/io_uring/cmd.h b/include/linux/io_uring/cmd.h
> > index 375fd048c4cb..424f071f42e5 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_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);
> >  #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_cmd_buf_ring_pin(struct io_uring_cmd *ioucmd,
> > +                                           unsigned buf_group,
> > +                                           unsigned issue_flags,
> > +                                           struct io_buffer_list **bl)
> > +{
> > +       return -EOPNOTSUPP;
> > +}
> > +static inline int io_uring_cmd_buf_ring_unpin(struct io_uring_cmd *ioucmd,
> > +                                             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 8f63924bc9f7..03e05bab023a 100644
> > --- a/io_uring/kbuf.c
> > +++ b/io_uring/kbuf.c
> > @@ -238,6 +238,50 @@ struct io_br_sel io_buffer_select(struct io_kiocb *req, size_t *len,
> >         return sel;
> >  }
> >
> > +int io_kbuf_ring_pin(struct io_kiocb *req, unsigned buf_group,
> > +                    unsigned issue_flags, struct io_buffer_list **bl)
> > +{
> > +       struct io_buffer_list *buffer_list;
> > +       struct io_ring_ctx *ctx = req->ctx;
>
> Would it make sense to take struct io_ring_ctx *ctx instead of struct
> io_kiocb *req since this doesn't otherwise use the req? Same comment
> for several of the other kbuf_ring helpers (io_kbuf_ring_unpin(),
> io_reg_buf_index_get(), io_reg_buf_index_put(), io_is_kmbuf_ring()).
>

I think this makes sense and is a good idea. I'll make this change for v4.

Thanks,
Joanne

> Best,
> Caleb
>