linux-kernel - Re: [PATCH] Revert "mm/page_alloc: make should_fail_alloc

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Date:   Mon, 5 Jul 2021 13:44:44 +0200
From:   Marco Elver <elver@...gle.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     akpm@...ux-foundation.org, glider@...gle.com, dvyukov@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kasan-dev@...glegroups.com, Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Yang Shi <shy828301@...il.com>, bpf@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH] Revert "mm/page_alloc: make should_fail_alloc_page()
 static"

On Mon, Jul 05, 2021 at 12:37PM +0100, Mel Gorman wrote:
> On Mon, Jul 05, 2021 at 12:38:06PM +0200, Marco Elver wrote:
> > This reverts commit f7173090033c70886d925995e9dfdfb76dbb2441.
> > 
> > Commit 76cd61739fd1 ("mm/error_inject: Fix allow_error_inject function
> > signatures") explicitly made should_fail_alloc_page() non-static, due to
> > worries of remaining compiler optimizations in the absence of function
> > side-effects while being noinline.
> > 
> > Furthermore, kernel/bpf/verifier.c pushes should_fail_alloc_page onto
> > the btf_non_sleepable_error_inject BTF IDs set, which when enabling
> > CONFIG_DEBUG_INFO_BTF results in an error at the BTFIDS stage:
> > 
> >   FAILED unresolved symbol should_fail_alloc_page
> > 
> > To avoid the W=1 warning, add a function declaration right above the
> > function itself, with a comment it is required in a BTF IDs set.
> > 
> > Fixes: f7173090033c ("mm/page_alloc: make should_fail_alloc_page() static")
> > Cc: Mel Gorman <mgorman@...hsingularity.net>
> > Cc: Alexei Starovoitov <ast@...nel.org>
> > Signed-off-by: Marco Elver <elver@...gle.com>
> 
> Acked-by: Mel Gorman <mgorman@...hsingularity.net>
> 
> Out of curiousity though, why does block/blk-core.c not require
> something similar for should_fail_bio?

It seems kernel/bpf/verifier.c doesn't refer to it in an BTF IDs set.
Looks like should_fail_alloc_page is special for BPF purposes. I'm not a
BPF maintainer, so hopefully someone can explain why
should_fail_alloc_page is special for BPF.

Thanks,
-- Marco