linux-kernel - Re: [PATCH bpf-next v5 1/3] btf: allow mmap of vmlinux btf

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Message-ID: <CAADnVQ+WZsaDS-Vuc9AN7P3=xvX8TG=rY65A8wYdOARLtkt6Mw@mail.gmail.com>
Date: Thu, 17 Jul 2025 07:48:59 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Lorenz Bauer <lmb@...valent.com>
Cc: Breno Leitao <leitao@...ian.org>, 
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, Arnd Bergmann <arnd@...db.de>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	linux-arch <linux-arch@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	bpf <bpf@...r.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, Alan Maguire <alan.maguire@...cle.com>
Subject: Re: [PATCH bpf-next v5 1/3] btf: allow mmap of vmlinux btf

On Thu, Jul 17, 2025 at 6:18 AM Lorenz Bauer <lmb@...valent.com> wrote:
>
> Hi Breno,
>
> Thanks for reaching out.
>
> On Thu, Jul 17, 2025 at 1:39 PM Breno Leitao <leitao@...ian.org> wrote:
>
> > Should __pa_symbol() be used instead of virt_to_phys()?
>
> I'm not really well versed with mm in general. Looking around a bit I
> found some explanation in [1]. Your suggested fix does make sense to
> me based on that.
>
> Let me run the patch against bpf-ci and see what happens.
>
> 1: https://lore.kernel.org/all/90667b2b7f773308318261f96ebefd1a67133c4c.1732464395.git.lukas@wunner.de/

Thanks for the link.
Key quote: "arm64 maps the kernel in the vmalloc space."
I think the map shouldn't be destroying linearity of kernel rodata.
__pa_symbol() should work for start_BTF, but would be good
to double check with Ard that the rest stays linear.