linux-kernel - Re: [BUG] WARNING: CPU: 3 PID: 1 at mm/debug_vm

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Date:   Fri, 22 Oct 2021 09:34:15 -1000
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Gavin Shan <gshan@...hat.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [BUG] WARNING: CPU: 3 PID: 1 at mm/debug_vm_pgtable.c:493

On Fri, Oct 22, 2021 at 2:38 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> It finally triggered again. And this time with this patch applied. But I
> don't see the added printks anywhere in the dmesg.

That's strange. Those printk's were added in the only places that do a
"return 0".

Ok, there's also the dummy pud_set_huge() inline function that
unconditionally returns zero, but that's only if you don't have
CONFIG_HAVE_ARCH_HUGE_VMAP enabled. And then the testing code is
disabled too.

> [  178.714431] debug_vm_pgtable: [debug_vm_pgtable         ]: Validating architecture page table helpers
> [  178.723726] ------------[ cut here ]------------
> [  178.728389] WARNING: CPU: 2 PID: 1 at mm/debug_vm_pgtable.c:492 pud_huge_tests+0x42/0x68

That's literally that

    WARN_ON(!pud_set_huge(..));

and pud_set_huge() has two 'return 0' (and one 'return 1') and that
patch added debug-printing to both of them.

Oh, it shouldn't have been a pr_debug() that gets suppressed. It
should have been a pr_warn() or something.

My bad.

                   Linus