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Subject: Re: [LKP] Re: [mm/debug] fa6726c1e7: kernel_BUG_at_include/linux/mm.h
To:     Catalin Marinas <catalin.marinas@arm.com>,
        "Chen, Rong A" <rong.a.chen@intel.com>
Cc:     Anshuman Khandual <anshuman.khandual@arm.com>,
        Qian Cai <cai@lca.pw>, kernel test robot <lkp@intel.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Ingo Molnar <mingo@kernel.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Vineet Gupta <vgupta@synopsys.com>,
        Will Deacon <will@kernel.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, lkp@lists.01.org
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 <813D7CD3-F31C-4056-92DF-D462633E9D69@lca.pw> <20200428092105.GB3868@gaia>
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 <07ea0efd-0145-eaaf-c628-e48957154a2c@intel.com>
 <20200429181550.GF10651@gaia>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <410b5235-bbd0-6cff-c361-c25665a854ae@c-s.fr>
Date:   Wed, 29 Apr 2020 20:34:17 +0200
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Le 29/04/2020 à 20:15, Catalin Marinas a écrit :
> On Wed, Apr 29, 2020 at 08:52:25PM +0800, Chen, Rong A wrote:
>> On 4/29/2020 11:28 AM, Anshuman Khandual wrote:
>>> On 04/28/2020 02:51 PM, Catalin Marinas wrote:
>>>> 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@arm.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.
>>> Could some one from LKP test framework, please confirm if this still causes
>>> above problem on the latest linux-next by default ?
>>
>> The .config is a rand config, the problem is still exist if run "make
>> oldconfig" for the config with commit 0a8646638865.
> 
> Is randconfig expected to boot? I don't think it is but I guess it
> should not trigger a BUG_ON during boot.
> 
>> $ grep -e CONFIG_MMU= -e CONFIG_EXPERT= -e CONFIG_ARCH_HAS_DEBUG_VM_PGTABLE=
>> -e CONFIG_DEBUG_VM= .config
>> CONFIG_EXPERT=y
>> CONFIG_MMU=y
>> CONFIG_DEBUG_VM=y
>>
>> should we disable DEBUG_VM_PGTABLE by default?
> 
> If that's the only case where this fails in LKP, I'd rather remove the
> EXPERT dependency so that it cannot be enabled. Architectures that want
> to experiment with this feature will have to select
> ARCH_HAS_DEBUG_VM_PGTABLE explicitly.
> 

But when something is not selectable, people won't even know it exists.

Why not try and fix the problems reported by the robots instead ?