Re: [PATCH v3 5/5] io_uring/rsrc: add io_uring_registered_mem_region_get()

[Joanne Koong <joannelkoong@gmail.com>]

On Wed, Mar 25, 2026 at 7:56 AM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 3/24/26 4:14 PM, Joanne Koong wrote:
> > diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
> > index cf5638406a0c..c706324fd66d 100644
> > --- a/io_uring/rsrc.c
> > +++ b/io_uring/rsrc.c
> > @@ -1182,6 +1182,24 @@ int io_import_reg_buf(struct io_kiocb *req, struct iov_iter *iter,
> >       return io_import_fixed(ddir, iter, node->buf, buf_addr, len);
> >  }
> >
> > +void *io_uring_registered_mem_region_get(struct io_uring_cmd *cmd,
> > +                                      unsigned *nr_pages,
> > +                                      unsigned issue_flags)
> > +{
> > +     struct io_ring_ctx *ctx = cmd_to_io_kiocb(cmd)->ctx;
> > +     void *ptr;
> > +
> > +     io_ring_submit_lock(ctx, issue_flags);
> > +
> > +     ptr = ctx->param_region.ptr;
> > +     *nr_pages = ctx->param_region.nr_pages;
> > +
> > +     io_ring_submit_unlock(ctx, issue_flags);
> > +
> > +     return ptr;
> > +}
> > +EXPORT_SYMBOL_GPL(io_uring_registered_mem_region_get);
>
> This looks suspicious, but I actually think it looks suspicious because
> you add the submit locking around it. For patterns like that, it makes
> the brain go "hmm, what protects this from going invalid the instant
> io_ring_submit_unlock() is called??". But this should be stable for the
> duration of the ring, hence the locking should not be needed at all?

My understanding is that once a memory region is registered to the
ring, it's registered for the ring's lifetime. There's no uapi to
unregister a memory region and my interpretation of the last paragraph
in this thread [1] is that unregistration is not intended to be
added/supported. I think the submit locking is needed in case another
thread is currently registering it so we don't see partially
initialized state between ptr and nr_pages (eg if the caller calls
this from a task work callback).

[1] https://lore.kernel.org/linux-fsdevel/000f7db7-5546-4680-bef2-84ce740ad8fd@gmail.com/

>
> I'd probably also prefer wrapping this in some kind of return struct, so
> that you just return that rather than having one part returned directly
> and one stuffed in a pointer to an unsigned for number of pages.

Sounds good, I will make this change.

Thanks,
Joanne
>
> --
> Jens Axboe