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

From: Mel Gorman <[email-masked]>
Date: Thu, 27 May 2021 15:54:41 +0100

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