Re: [LKP] Re: [mm/debug] fa6726c1e7: kernel_BUG_at_include/linux/mm.h

Christophe Leroy
2020-04-29T18:35:25Z

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 ?