linux-kernel - Re: [mm/debug] fa6726c1e7: kernel_BUG_at

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Message-ID: <20200428092105.GB3868@gaia>
Date:   Tue, 28 Apr 2020 10:21:05 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Qian Cai <cai@....pw>
Cc:     Anshuman Khandual <Anshuman.Khandual@....com>,
        Christophe Leroy <christophe.leroy@....fr>,
        kernel test robot <lkp@...el.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Ingo Molnar <mingo@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Vineet Gupta <vgupta@...opsys.com>,
        Will Deacon <will@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org
Subject: Re: [mm/debug] fa6726c1e7: kernel_BUG_at_include/linux/mm.h

On Tue, Apr 28, 2020 at 04:41:11AM -0400, Qian Cai wrote:
> On Apr 28, 2020, at 1:54 AM, Anshuman Khandual <Anshuman.Khandual@....com> wrote:
> > That is true. There is a slight change in the rules, making it explicit yes
> > only when both ARCH_HAS_DEBUG_VM_PGTABLE and DEBUG_VM are enabled.
> > 
> > +config DEBUG_VM_PGTABLE
> > +    bool "Debug arch page table for semantics compliance"
> > +    depends on MMU
> > +    depends on !IA64 && !ARM
> > +    depends on ARCH_HAS_DEBUG_VM_PGTABLE || EXPERT
> > +    default y if ARCH_HAS_DEBUG_VM_PGTABLE && DEBUG_VM
> > +    help
> > 
> > The default is really irrelevant as the config option can be set explicitly.
> 
> That could also explain. Since not long time ago, it was only “default
> y if DEBUG_VM”, that caused the robot saved a .config with
> DEBUG_VM_PGTABLE=y by default.
> 
> Even though you changed the rule recently, it has no effect as the
> robot could “make oldconfig” from the saved config for each linux-next
> tree execution and the breakage will go on.

I'm not entirely sure that's the case. This report still points at the
old commit fa6726c1e7 which has:

+       depends on ARCH_HAS_DEBUG_VM_PGTABLE || EXPERT
+       default n if !ARCH_HAS_DEBUG_VM_PGTABLE
+       default y if DEBUG_VM

In -next we now have commit 647d9a0de34c and subsequently modified by
commit 0a8646638865. So hopefully with the latest -next tree we won't
see this report.

We could as well remove the 'depends on ... || EXPERT' part but I'd
rather leave this around with a default n (as in current -next) in case
others want to have a go. If that's still causing problems, we can
remove the '|| EXPERT' part, so there won't be any further regressions.

-- 
Catalin