Re: [PATCH v1 06/30] io_uring/kbuf: add buffer ring pinning/unpinning

[Joanne Koong <joannelkoong@gmail.com>]

On Tue, Dec 2, 2025 at 8:13 PM Caleb Sander Mateos
<csander@purestorage.com> wrote:
>
> On Tue, Dec 2, 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.
> > Pinning an already-pinned bufring is acceptable and returns 0.
> >
> > The API accepts a "struct io_ring_ctx *ctx" rather than a cmd pointer,
> > as the buffer ring may need to be unpinned in contexts where a cmd is
> > not readily available.
> >
> > 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/buf.h | 28 +++++++++++++++++++++++
> >  io_uring/kbuf.c              | 43 ++++++++++++++++++++++++++++++++++++
> >  io_uring/kbuf.h              |  5 +++++
> >  3 files changed, 76 insertions(+)
> >  create mode 100644 include/linux/io_uring/buf.h
> >
> > diff --git a/io_uring/kbuf.c b/io_uring/kbuf.c
> > index 00ab17a034b5..ddda1338e652 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/buf.h>
> >
> >  #include <uapi/linux/io_uring.h>
> >
> > @@ -237,6 +238,46 @@ struct io_br_sel io_buffer_select(struct io_kiocb *req, size_t *len,
> >         return sel;
> >  }
> >
> > +int io_uring_buf_ring_pin(struct io_ring_ctx *ctx, unsigned buf_group,
> > +                         unsigned issue_flags, struct io_buffer_list **bl)
> > +{
> > +       struct io_buffer_list *buffer_list;
> > +       int ret = -EINVAL;
> > +
> > +       io_ring_submit_lock(ctx, issue_flags);
> > +
> > +       buffer_list = io_buffer_get_list(ctx, buf_group);
> > +       if (likely(buffer_list) && (buffer_list->flags & IOBL_BUF_RING)) {
>
> Since there's no reference-counting of pins, I think it might make
> more sense to fail io_uring_buf_ring_pin() if the buffer ring is
> already pinned. Otherwise, the buffer ring will be unpinned in the
> first call to io_uring_buf_ring_unpin(), when it might still be in use
> by another caller of io_uring_buf_ring_pin().

That makes sense, I'll change this to return -EALREADY then if it's
already pinned.

Thanks,
Joanne
>
> Best,
> Caleb
>