Re: [PATCH bpf-next v1 2/8] bpf: Add bpf_page_to_pfn helper

From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
To: Yonghong Song <yhs@fb.com>
Cc: bpf@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Pavel Emelyanov <ovzxemul@gmail.com>,
	Alexander Mihalicyn <alexander@mihalicyn.com>,
	Andrei Vagin <avagin@gmail.com>,
	criu@openvz.org, io-uring@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH bpf-next v1 2/8] bpf: Add bpf_page_to_pfn helper
Date: Fri, 19 Nov 2021 00:00:22 +0530	[thread overview]
Message-ID: <20211118183022.tiijkhno57uwtytm@apollo.localdomain> (raw)
In-Reply-To: <7efcf912-46be-1ed4-7e12-88c2baeaab12@fb.com>

On Thu, Nov 18, 2021 at 10:57:15PM IST, Yonghong Song wrote:
>
>
> On 11/15/21 9:42 PM, Kumar Kartikeya Dwivedi wrote:
> > 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],
>
> Does this helper need to be gpl_only?
>

Not sure about it, it wraps over a macro.

> > +};
> > +
> >   static int __init io_uring_iter_init(void)
> >   {
> >   	io_uring_buf_reg_info.ctx_arg_info[0].btf_id = btf_io_uring_ids[0];
> [...]

--
Kartikeya