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

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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]

base: next
config: ia64-allmodconfig (attached as .config)
compiler: ia64-linux-gcc (GCC) 7.5.0
          wget -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
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.

	bool "Debug arch page table for semantics compliance"
	depends on MMU
	depends on !(IA64 || ARM)
	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.


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