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

[Christophe Leroy]

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 at arm.com> 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 at intel.com>
>>>>>>
>>>>>> 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.

Christophe