Re: [PATCH V13] mm/debug: Add tests validating architecture page table helpers

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On 02/10/2020 04:36 PM, Russell King - ARM Linux admin wrote:
> On Mon, Feb 10, 2020 at 11:46:23AM +0100, Christophe Leroy wrote:
>>
>>
>> Le 10/02/2020 à 11:02, Russell King - ARM Linux admin a écrit :
>>> On Mon, Feb 10, 2020 at 07:38:38AM +0100, Christophe Leroy wrote:
>>>>
>>>>
>>>> Le 10/02/2020 à 06:35, Anshuman Khandual a écrit :
>>>>>
>>>>>
>>>>> On 02/10/2020 10:22 AM, Andrew Morton wrote:
>>>>>> On Thu, 6 Feb 2020 13:49:35 +0530 Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
>>>>>>
>>>>>>>
>>>>>>> On 02/06/2020 04:40 AM, kbuild test robot wrote:
>>>>>>>> Hi Anshuman,
>>>>>>>>
>>>>>>>> Thank you for the patch! Yet something to improve:
>>>>>>>>
>>>>>>>> [auto build test ERROR on powerpc/next]
>>>>>>>> [also build test ERROR on s390/features linus/master arc/for-next v5.5]
>>>>>>>> [cannot apply to mmotm/master tip/x86/core arm64/for-next/core next-20200205]
>>>>>>>> [if your patch is applied to the wrong git tree, please drop us a note to help
>>>>>>>> improve the system. BTW, we also suggest to use '--base' option to specify the
>>>>>>>> base tree in git format-patch, please see https://stackoverflow.com/a/37406982]
>>>>>>>>
>>>>>>>> url:    https://github.com/0day-ci/linux/commits/Anshuman-Khandual/mm-debug-Add-tests-validating-architecture-page-table-helpers/20200205-215507
>>>>>>>> base:   https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git next
>>>>>>>> config: ia64-allmodconfig (attached as .config)
>>>>>>>> compiler: ia64-linux-gcc (GCC) 7.5.0
>>>>>>>> reproduce:
>>>>>>>>           wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>>>>>>>>           chmod +x ~/bin/make.cross
>>>>>>>>           # save the attached .config to linux build tree
>>>>>>>>           GCC_VERSION=7.5.0 make.cross ARCH=ia64
>>>>>>>>
>>>>>>>> If you fix the issue, kindly add following tag
>>>>>>>> Reported-by: kbuild test robot <lkp@xxxxxxxxx>
>>>>>>>>
>>>>>>>> All error/warnings (new ones prefixed by >>):
>>>>>>>>
>>>>>>>>      In file included from include/asm-generic/pgtable-nopud.h:8:0,
>>>>>>>>                       from arch/ia64/include/asm/pgtable.h:586,
>>>>>>>>                       from include/linux/mm.h:99,
>>>>>>>>                       from include/linux/highmem.h:8,
>>>>>>>>                       from mm/debug_vm_pgtable.c:14:
>>>>>>>>      mm/debug_vm_pgtable.c: In function 'pud_clear_tests':
>>>>>>>>>> include/asm-generic/pgtable-nop4d-hack.h:47:32: error: implicit declaration of function '__pgd'; did you mean '__p4d'? [-Werror=implicit-function-declaration]
>>>>>>>>       #define __pud(x)    ((pud_t) { __pgd(x) })
>>>>>>>>                                      ^
>>>>>>>>>> mm/debug_vm_pgtable.c:141:8: note: in expansion of macro '__pud'
>>>>>>>>        pud = __pud(pud_val(pud) | RANDOM_ORVALUE);
>>>>>>>>              ^~~~~
>>>>>>>>>> include/asm-generic/pgtable-nop4d-hack.h:47:22: warning: missing braces around initializer [-Wmissing-braces]
>>>>>>>>       #define __pud(x)    ((pud_t) { __pgd(x) })
>>>>>>>>                            ^
>>>>>>>>>> mm/debug_vm_pgtable.c:141:8: note: in expansion of macro '__pud'
>>>>>>>>        pud = __pud(pud_val(pud) | RANDOM_ORVALUE);
>>>>>>>>              ^~~~~
>>>>>>>>      cc1: some warnings being treated as errors
>>>>>>>
>>>>>>> This build failure is expected now given that we have allowed DEBUG_VM_PGTABLE
>>>>>>> with EXPERT without platform requiring ARCH_HAS_DEBUG_VM_PGTABLE. This problem
>>>>>>> i.e build failure caused without a platform __pgd(), is known to exist both on
>>>>>>> ia64 and arm (32bit) platforms. Please refer https://lkml.org/lkml/2019/9/24/314
>>>>>>> for details where this was discussed earlier.
>>>>>>>
>>>>>>
>>>>>> I'd prefer not to merge a patch which is known to cause build
>>>>>> regressions.  Is there some temporary thing we can do to prevent these
>>>>>> errors until arch maintainers(?) get around to implementing the
>>>>>> long-term fixes?
>>>>>
>>>>> We could explicitly disable CONFIG_DEBUG_VM_PGTABLE on ia64 and arm platforms
>>>>> which will ensure that others can still use the EXPERT path.
>>>>>
>>>>> 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 n if !ARCH_HAS_DEBUG_VM_PGTABLE
>>>>> 	default y if DEBUG_VM
>>>>>
>>>>
>>>> On both ia32 and arm, the fix is trivial.
>>>>
>>>> Can we include the fix within this patch, just the same way as the
>>>> mm_p4d_folded() fix for x86 ?
>>>
>>> Why should arm include a macro for something that nothing (apart from
>>> this checker) requires?  If the checker requires it but the rest of
>>> the kernel does not, it suggests that the checker isn't actually
>>> correct, and the results can't be relied upon.
>>>
>>
>> As far as I can see, the problem is that arm opencodes part of the API
>> instead of including asm-generic/pgtable-nopmd.h
>>
>> Here, the ARM has 2 levels, ie only PGD and PTE. But instead of defining
>> __pgd and __pte and getting everything else from asm-generic, it defines a
>> __pmd then redefines the folded levels like the pud, etc ...
>>
>> That's exactly what the checker aims at detecting: architectures than do not
>> properly use the standard linux page table structures.
> 
> There are good reasons for the way ARM does stuff.  The generic crap was
> written without regard for the circumstances that ARM has, and thus is
> entirely unsuitable for 32-bit ARM.

Since we dont have an agreement here, lets just settle with disabling the
test for now on platforms where the build fails. CONFIG_EXPERT is enabling
this test for better adaptability and coverage, hence how about re framing
the config like this ? This at the least conveys the fact that EXPERT only
works when platform is neither IA64 or ARM.

config DEBUG_VM_PGTABLE
	bool "Debug arch page table for semantics compliance"
	depends on MMU
	depends on ARCH_HAS_DEBUG_VM_PGTABLE || (EXPERT &&  !(IA64 || ARM))
	default n if !ARCH_HAS_DEBUG_VM_PGTABLE
	default y if DEBUG_VM





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