linux-kernel - Re: [PATCH] x86/numa: Use cpumask_available instead of hardcoded NULL check

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Date:   Tue, 02 Aug 2022 21:59:05 +0530
From:   Siddh Raman Pant <code@...dh.me>
To:     "Ingo Molnar" <mingo@...nel.org>
Cc:     "x86" <x86@...nel.org>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Andy Lutomirski" <luto@...nel.org>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-kernel" <linux-kernel@...r.kernel.org>,
        "linux-kernel-mentees" 
        <linux-kernel-mentees@...ts.linuxfoundation.org>
Subject: Re: [PATCH] x86/numa: Use cpumask_available instead of hardcoded
 NULL check

On Tue, 02 Aug 2022 16:37:44 +0530  Ingo Molnar <mingo@...nel.org> wrote:
> Your fix makes sense I suppose, but I'm wondering how testing didn't 
> trigger this warning.
> 
> Off-stack isn't a rare config option:
> 
>   kepler:~/tip> make allmodconfig
>   #
>   # No change to .config
>   #
>   kepler:~/tip> grep CPUMASK_OFFSTACK .config
>   CONFIG_CPUMASK_OFFSTACK=y
>   kepler:~/tip> 
> 
> What am I missing?

Maybe this triggers on certain config options set, or maybe due to new
gcc version? (I'm using gcc-12, I also likely saw while on gcc-11.)
It nevertheless is a helpful warning.

I just now tried `make defconfig` (default configuration based on
'x86_64_defconfig') and compiling with `make -j13 all`, and gcc doesn't
give any warning. (CONFIG_CPUMASK_OFFSTACK isn't even listed in the
.config file produced, grep fails.)

The config on which I can reproduce the warning can be found here:
https://gist.github.com/siddhpant/0197ea2b9873e8719d5d7ef991e2cd89
(It has 8969 lines, thus uploaded as a gist.)

This is a modification of a config found on syzkaller, which I was using
to compile and test some bug. I had noticed the gcc warning earlier while
on similar detours and usually ignored it, but now I finally took a look.

I tested compiling with it 5 times (`make clean` and `make -j13 all`),
and gcc gave the warning in all attempts. I also tried `make -j1 all`,
which also had gcc spitting out the warning, so it cannot be any race.

> > Fixes: c032ef60d1aa ("cpumask: convert node_to_cpumask_map[] to cpumask_var_t")
> > Fixes: de2d9445f162 ("x86: Unify node_to_cpumask_map handling between 32 and 64bit")
> 
> These are ancient commits from 2009 & 2011.

Yes, that's where blaming the file leads me to.

Thanks,
Siddh