Re: [PATCH 1/1] mm/pgtable/debug: Add test validating architecture page table helpers

[Anshuman Khandual]

On 09/09/2019 08:43 PM, Kirill A. Shutemov wrote:
> On Mon, Sep 09, 2019 at 11:56:50AM +0530, Anshuman Khandual wrote:
>>
>>
>> On 09/07/2019 12:33 AM, Gerald Schaefer wrote:
>>> On Fri, 6 Sep 2019 11:58:59 +0530
>>> Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
>>>
>>>> On 09/05/2019 10:36 PM, Gerald Schaefer wrote:
>>>>> On Thu, 5 Sep 2019 14:48:14 +0530
>>>>> Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
>>>>>   
>>>>>>> [...]    
>>>>>>>> +
>>>>>>>> +#if !defined(__PAGETABLE_PMD_FOLDED) && !defined(__ARCH_HAS_4LEVEL_HACK)
>>>>>>>> +static void pud_clear_tests(pud_t *pudp)
>>>>>>>> +{
>>>>>>>> +	memset(pudp, RANDOM_NZVALUE, sizeof(pud_t));
>>>>>>>> +	pud_clear(pudp);
>>>>>>>> +	WARN_ON(!pud_none(READ_ONCE(*pudp)));
>>>>>>>> +}    
>>>>>>>
>>>>>>> For pgd/p4d/pud_clear(), we only clear if the page table level is present
>>>>>>> and not folded. The memset() here overwrites the table type bits, so
>>>>>>> pud_clear() will not clear anything on s390 and the pud_none() check will
>>>>>>> fail.
>>>>>>> Would it be possible to OR a (larger) random value into the table, so that
>>>>>>> the lower 12 bits would be preserved?    
>>>>>>
>>>>>> So the suggestion is instead of doing memset() on entry with RANDOM_NZVALUE,
>>>>>> it should OR a large random value preserving lower 12 bits. Hmm, this should
>>>>>> still do the trick for other platforms, they just need non zero value. So on
>>>>>> s390, the lower 12 bits on the page table entry already has valid value while
>>>>>> entering this function which would make sure that pud_clear() really does
>>>>>> clear the entry ?  
>>>>>
>>>>> Yes, in theory the table entry on s390 would have the type set in the last
>>>>> 4 bits, so preserving those would be enough. If it does not conflict with
>>>>> others, I would still suggest preserving all 12 bits since those would contain
>>>>> arch-specific flags in general, just to be sure. For s390, the pte/pmd tests
>>>>> would also work with the memset, but for consistency I think the same logic
>>>>> should be used in all pxd_clear_tests.  
>>>>
>>>> Makes sense but..
>>>>
>>>> There is a small challenge with this. Modifying individual bits on a given
>>>> page table entry from generic code like this test case is bit tricky. That
>>>> is because there are not enough helpers to create entries with an absolute
>>>> value. This would have been easier if all the platforms provided functions
>>>> like __pxx() which is not the case now. Otherwise something like this should
>>>> have worked.
>>>>
>>>>
>>>> pud_t pud = READ_ONCE(*pudp);
>>>> pud = __pud(pud_val(pud) | RANDOM_VALUE (keeping lower 12 bits 0))
>>>> WRITE_ONCE(*pudp, pud);
>>>>
>>>> But __pud() will fail to build in many platforms.
>>>
>>> Hmm, I simply used this on my system to make pud_clear_tests() work, not
>>> sure if it works on all archs:
>>>
>>> pud_val(*pudp) |= RANDOM_NZVALUE;
>>
>> Which compiles on arm64 but then fails on x86 because of the way pmd_val()
>> has been defined there.
> 
> Use instead
> 
> 	*pudp = __pud(pud_val(*pudp) | RANDOM_NZVALUE);

Agreed.

As I had mentioned before this would have been really the cleanest approach.

> 
> It *should* be more portable.

Not really, because not all the platforms have __pxx() definitions right now.
Going with these will clearly cause build failures on affected platforms. Lets
examine __pud() for instance. It is defined only on these platforms.

arch/arm64/include/asm/pgtable-types.h:		#define __pud(x) ((pud_t) { (x) } )
arch/mips/include/asm/pgtable-64.h:		#define __pud(x) ((pud_t) { (x) })
arch/powerpc/include/asm/pgtable-be-types.h:	#define __pud(x) ((pud_t) { cpu_to_be64(x) })
arch/powerpc/include/asm/pgtable-types.h:	#define __pud(x) ((pud_t) { (x) })
arch/s390/include/asm/page.h:			#define __pud(x) ((pud_t) { (x) } )
arch/sparc/include/asm/page_64.h:		#define __pud(x) ((pud_t) { (x) } )
arch/sparc/include/asm/page_64.h:		#define __pud(x) (x)
arch/x86/include/asm/pgtable.h:			#define __pud(x) native_make_pud(x)

Similarly for __pmd()

arch/alpha/include/asm/page.h:			#define __pmd(x)  ((pmd_t) { (x) } )
arch/arm/include/asm/page-nommu.h:		#define __pmd(x)  (x)
arch/arm/include/asm/pgtable-2level-types.h:	#define __pmd(x)  ((pmd_t) { (x) } )
arch/arm/include/asm/pgtable-2level-types.h:	#define __pmd(x)  (x)
arch/arm/include/asm/pgtable-3level-types.h:	#define __pmd(x)  ((pmd_t) { (x) } )
arch/arm/include/asm/pgtable-3level-types.h:	#define __pmd(x)  (x)
arch/arm64/include/asm/pgtable-types.h:		#define __pmd(x)  ((pmd_t) { (x) } )
arch/m68k/include/asm/page.h:			#define __pmd(x)  ((pmd_t) { { (x) }, })
arch/mips/include/asm/pgtable-64.h:		#define __pmd(x)  ((pmd_t) { (x) } )
arch/nds32/include/asm/page.h:			#define __pmd(x)  (x)
arch/parisc/include/asm/page.h:			#define __pmd(x)  ((pmd_t) { (x) } )
arch/parisc/include/asm/page.h:			#define __pmd(x)  (x)
arch/powerpc/include/asm/pgtable-be-types.h:	#define __pmd(x)  ((pmd_t) { cpu_to_be64(x) })
arch/powerpc/include/asm/pgtable-types.h:	#define __pmd(x)  ((pmd_t) { (x) })
arch/riscv/include/asm/pgtable-64.h:		#define __pmd(x)  ((pmd_t) { (x) })
arch/s390/include/asm/page.h:			#define __pmd(x)  ((pmd_t) { (x) } )
arch/sh/include/asm/pgtable-3level.h:		#define __pmd(x)  ((pmd_t) { (x) } )
arch/sparc/include/asm/page_32.h:		#define __pmd(x)  ((pmd_t) { { (x) }, })
arch/sparc/include/asm/page_32.h:		#define __pmd(x)  ((pmd_t) { { (x) }, })
arch/sparc/include/asm/page_64.h:		#define __pmd(x)  ((pmd_t) { (x) } )
arch/sparc/include/asm/page_64.h:		#define __pmd(x)  (x)
arch/um/include/asm/page.h:			#define __pmd(x)  ((pmd_t) { (x) } )
arch/um/include/asm/page.h:			#define __pmd(x)  ((pmd_t) { (x) } )
arch/x86/include/asm/pgtable.h:			#define __pmd(x)  native_make_pmd(x)

Similarly for __pgd()

arch/alpha/include/asm/page.h:			#define __pgd(x)  ((pgd_t) { (x) } )
arch/alpha/include/asm/page.h:			#define __pgd(x)  (x)
arch/arc/include/asm/page.h:			#define __pgd(x)  ((pgd_t) { (x) })
arch/arc/include/asm/page.h:			#define __pgd(x)  (x)
arch/arm/include/asm/pgtable-3level-types.h:	#define __pgd(x)  ((pgd_t) { (x) } )
arch/arm/include/asm/pgtable-3level-types.h:	#define __pgd(x)  (x)
arch/arm64/include/asm/pgtable-types.h:		#define __pgd(x)  ((pgd_t) { (x) } )
arch/csky/include/asm/page.h:			#define __pgd(x)  ((pgd_t) { (x) })
arch/hexagon/include/asm/page.h:		#define __pgd(x)  ((pgd_t) { (x) })
arch/m68k/include/asm/page.h:			#define __pgd(x)  ((pgd_t) { (x) } )
arch/mips/include/asm/page.h:			#define __pgd(x)  ((pgd_t) { (x) } )
arch/nds32/include/asm/page.h:			#define __pgd(x)  (x)
arch/nios2/include/asm/page.h:			#define __pgd(x)  ((pgd_t) { (x) })
arch/openrisc/include/asm/page.h:		#define __pgd(x)  ((pgd_t) { (x) })
arch/parisc/include/asm/page.h:			#define __pgd(x)  ((pgd_t) { (x) } )
arch/parisc/include/asm/page.h:			#define __pgd(x)  (x)
arch/powerpc/include/asm/pgtable-be-types.h:	#define __pgd(x)  ((pgd_t) { cpu_to_be64(x) })
arch/powerpc/include/asm/pgtable-types.h:	#define __pgd(x)  ((pgd_t) { (x) })
arch/riscv/include/asm/page.h:			#define __pgd(x)  ((pgd_t) { (x) })
arch/s390/include/asm/page.h:			#define __pgd(x)  ((pgd_t) { (x) } )
arch/sh/include/asm/page.h:			#define __pgd(x)  ((pgd_t) { (x) } )
arch/sparc/include/asm/page_32.h:		#define __pgd(x)  ((pgd_t) { (x) } )
arch/sparc/include/asm/page_32.h:		#define __pgd(x)  (x)
arch/sparc/include/asm/page_64.h:		#define __pgd(x)  ((pgd_t) { (x) } )
arch/sparc/include/asm/page_64.h:		#define __pgd(x)  (x)
arch/um/include/asm/page.h:			#define __pgd(x)  ((pgd_t) { (x) } )
arch/unicore32/include/asm/page.h:		#define __pgd(x)  ((pgd_t) { (x) })
arch/unicore32/include/asm/page.h:		#define __pgd(x)  (x)
arch/x86/include/asm/pgtable.h:			#define __pgd(x)  native_make_pgd(x)
arch/xtensa/include/asm/page.h:			#define __pgd(x)  ((pgd_t) { (x) } )

Similarly for __p4d()

arch/s390/include/asm/page.h:			#define __p4d(x)  ((p4d_t) { (x) } )
arch/x86/include/asm/pgtable.h:			#define __p4d(x)  native_make_p4d(x)

The search pattern here has been "#define __pxx(". Unless I am missing something,
I dont see how we can use these without risking build failures.