linux-kernel - Re: [PATCH] mm/page_alloc: Mark pagesets as __maybe

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Date:   Wed, 16 Feb 2022 09:00:09 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        "kernelci.org bot" <bot@...nelci.org>
Subject: Re: [PATCH] mm/page_alloc: Mark pagesets as __maybe_unused

On 2022-02-15 11:43:22 [-0700], Nathan Chancellor wrote:
> Commit 9983a9d577db ("locking/local_lock: Make the empty local_lock_*()
> function a macro.") in the -tip tree converted the local_lock_*()
> functions into macros, which causes a warning with clang with
> CONFIG_PREEMPT_RT=n + CONFIG_DEBUG_LOCK_ALLOC=n:
> 
>   mm/page_alloc.c:131:40: error: variable 'pagesets' is not needed and will not be emitted [-Werror,-Wunneeded-internal-declaration]
>   static DEFINE_PER_CPU(struct pagesets, pagesets) = {
>                                          ^
>   1 error generated.
> 
> Prior to that change, clang was not able to tell that pagesets was
> unused in this configuration because it does not perform cross function
> analysis in the frontend. After that change, it sees that the macros
> just do a typecheck on the lock member of pagesets, which is evaluated
> at compile time (so the variable is technically "used"), meaning the
> variable is not needed in the final assembly, as the warning states.
> 
> Mark the variable as __maybe_unused to make it clear to clang that this
> is expected in this configuration so there is no more warning.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1593
> Reported-by: "kernelci.org bot" <bot@...nelci.org>
> Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> ---
>  mm/page_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7ff1efc84205..406f5d0c610f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -128,7 +128,7 @@ static DEFINE_MUTEX(pcp_batch_high_lock);
>  struct pagesets {
>  	local_lock_t lock;
>  };
> -static DEFINE_PER_CPU(struct pagesets, pagesets) = {
> +static DEFINE_PER_CPU(struct pagesets, pagesets) __maybe_unused = {

No, I need to think of something else then for the local_lock thing.  I
haven't seen it with gcc. There is probably more than just this one.

>  	.lock = INIT_LOCAL_LOCK(lock),
>  };

Sebastian