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

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Date:   Thu, 27 May 2021 09:04:24 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrii Nakryiko <andrii.nakryiko@...il.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>,
        open list <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Hritik Vijay <hritikxx8@...il.com>, bpf <bpf@...r.kernel.org>,
        Linux-Net <netdev@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH] mm/page_alloc: Work around a pahole limitation with
 zero-sized struct pagesets

On Wed, May 26, 2021 at 09:07:41AM +0100, Mel Gorman wrote:
> +    !defined(CONFIG_DEBUG_LOCK_ALLOC) &&		\
> +    !defined(CONFIG_PAHOLE_HAS_ZEROSIZE_PERCPU_SUPPORT)
> +	/*
> +	 * pahole 1.21 and earlier gets confused by zero-sized per-CPU
> +	 * variables and produces invalid BTF. Ensure that
> +	 * sizeof(struct pagesets) != 0 for older versions of pahole.
> +	 */
> +	char __pahole_hack;
> +	#warning "pahole too old to support zero-sized struct pagesets"
> +#endif

Err, hell no.  We should not mess up the kernel for broken tools that
are not relevant to the kernel build itself ever.