linux-kernel - Re: [PATCH v2] mm/page_alloc: Work around a pahole limitation with zero-sized struct pagesets

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Date:   Thu, 27 May 2021 15:54:41 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Michal Suchanek <msuchanek@...e.de>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Hritik Vijay <hritikxx8@...il.com>,
        Linux-BPF <bpf@...r.kernel.org>,
        Linux-Net <netdev@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v2] mm/page_alloc: Work around a pahole limitation with
 zero-sized struct pagesets

On Thu, May 27, 2021 at 07:37:05AM -0700, Andrii Nakryiko wrote:
> > This patch checks for older versions of pahole and only allows
> > DEBUG_INFO_BTF_MODULES if pahole supports zero-sized per-cpu structures.
> > DEBUG_INFO_BTF is still allowed as a KVM boot test passed with pahole
> 
> Unfortunately this won't work. The problem is that vmlinux BTF is
> corrupted, which results in module BTFs to be rejected as well, as
> they depend on it.
> 
> But vmlinux BTF corruption makes BPF subsystem completely unusable. So
> even though kernel boots, nothing BPF-related works. So we'd need to
> add dependency for DEBUG_INFO_BTF on pahole 1.22+.
> 

While bpf usage would be broken, the kernel will boot and the effect
should be transparent to any kernel build based on "make oldconfig".
CONFIG_DEBUG_INFO_BTF defaults N so if that is forced out, it will be
easily missed by a distribution kernel maintainer.

Yes, users of BPF will be affected and it may generate bug reports but
the fix will be to build with a working pahole. Breaking boot on the other
hand is a lot more visible and hacking around this with a non-zero struct
size has been shot down.

-- 
Mel Gorman
SUSE Labs