In CRIU, we need to be able to determine whether the page pinned by
io_uring is still present in the same range in the process VMA.
/proc/<pid>/pagemap gives us the PFN, hence using this helper we can
establish this mapping easily from the iterator side.

It is a simple wrapper over the in-kernel page_to_pfn helper, and
ensures the passed in pointer is a struct page PTR_TO_BTF_ID. This is
obtained from the bvec of io_uring_ubuf for the CRIU usecase.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
  fs/io_uring.c                  | 17 +++++++++++++++++
  include/linux/bpf.h            |  1 +
  include/uapi/linux/bpf.h       |  9 +++++++++
  kernel/trace/bpf_trace.c       |  2 ++
  scripts/bpf_doc.py             |  2 ++
  tools/include/uapi/linux/bpf.h |  9 +++++++++
  6 files changed, 40 insertions(+)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 46a110989155..9e9df6767e29 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c

@@ -11295,6 +11295,23 @@ static struct bpf_iter_reg io_uring_buf_reg_info = {
  	.seq_info	   = &bpf_io_uring_buf_seq_info,
  };
  
+BPF_CALL_1(bpf_page_to_pfn, struct page *, page)
+{
+	/* PTR_TO_BTF_ID can be NULL */
+	if (!page)
+		return U64_MAX;
+	return page_to_pfn(page);
+}
+
+BTF_ID_LIST_SINGLE(btf_page_to_pfn_ids, struct, page)
+
+const struct bpf_func_proto bpf_page_to_pfn_proto = {
+	.func		= bpf_page_to_pfn,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_BTF_ID,
+	.arg1_btf_id	= &btf_page_to_pfn_ids[0],