Re: [PATCH -next v5 3/5] mm: page_table_check: add hooks to public helpers

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 4/24/22 09:40, Tong Tiangen wrote:
> 
> 
> 在 2022/4/22 14:05, Anshuman Khandual 写道:
>>
>>
>> On 4/21/22 13:50, Tong Tiangen wrote:
>>> Move ptep_clear() to the include/linux/pgtable.h and add page table check
>>> relate hooks to some helpers, it's prepare for support page table check
>>> feature on new architecture.
>>
>> Could instrumenting generic page table helpers (fallback instances when its
>> corresponding __HAVE_ARCH_XXX is not defined on the platform), might add all
>> the page table check hooks into paths on platforms which have not subscribed
>> ARCH_SUPPORTS_PAGE_TABLE_CHECK in the first place ? Although these looks have
>> !CONFIG_PAGE_TABLE_CHECK fallback stubs in the header, hence a build problem
>> gets avoided.
> 
> Right, build problems are avoided by fallback stubs in the header file.

Although there might not be a build problem as such, but should non subscribing
platforms get their page table helpers instrumented with page table check hooks
in the first place ? The commit message should address these questions.

> 
>>
>>>
>>> Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>
>>> Acked-by: Pasha Tatashin <pasha.tatashin@soleen.com>
>>> ---
>>>   arch/x86/include/asm/pgtable.h | 10 ----------
>>>   include/linux/pgtable.h        | 26 ++++++++++++++++++--------
>>>   2 files changed, 18 insertions(+), 18 deletions(-)
>>>
>>> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
>>> index 564abe42b0f7..51cd39858f81 100644
>>> --- a/arch/x86/include/asm/pgtable.h
>>> +++ b/arch/x86/include/asm/pgtable.h
>>> @@ -1073,16 +1073,6 @@ static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
>>>       return pte;
>>>   }
>>>   -#define __HAVE_ARCH_PTEP_CLEAR
>>
>> AFICS X86 is the only platform subscribing __HAVE_ARCH_PTEP_CLEAR. Hence if
>> this is getting dropped for generic ptep_clear(), then no need to add back
>> #ifnded __HAVE_ARCH_PTEP_CLEAR construct. Generic ptep_clear() is the only
>> definition for all platforms ?
>>
>> Also if this patch is trying to drop off __HAVE_ARCH_PTEP_CLEAR along with
>> other page table check related changes, it needs to be done via a separate
>> patch instead.
> 
> Agreed.
> IMO, this fix can be patched later.
> 
>>
>>> -static inline void ptep_clear(struct mm_struct *mm, unsigned long addr,
>>> -                  pte_t *ptep)
>>> -{
>>> -    if (IS_ENABLED(CONFIG_PAGE_TABLE_CHECK))
>>> -        ptep_get_and_clear(mm, addr, ptep);
>>> -    else
>>> -        pte_clear(mm, addr, ptep);
>>> -}
>>> -
>>>   #define __HAVE_ARCH_PTEP_SET_WRPROTECT
>>>   static inline void ptep_set_wrprotect(struct mm_struct *mm,
>>>                         unsigned long addr, pte_t *ptep)
>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>> index 49ab8ee2d6d7..10d2d91edf20 100644
>>> --- a/include/linux/pgtable.h
>>> +++ b/include/linux/pgtable.h
>>> @@ -12,6 +12,7 @@
>>>   #include <linux/bug.h>
>>>   #include <linux/errno.h>
>>>   #include <asm-generic/pgtable_uffd.h>
>>> +#include <linux/page_table_check.h>
>>>     #if 5 - defined(__PAGETABLE_P4D_FOLDED) - defined(__PAGETABLE_PUD_FOLDED) - \
>>>       defined(__PAGETABLE_PMD_FOLDED) != CONFIG_PGTABLE_LEVELS
>>> @@ -272,14 +273,6 @@ static inline bool arch_has_hw_pte_young(void)
>>>   }
>>>   #endif
>>>   -#ifndef __HAVE_ARCH_PTEP_CLEAR
>>> -static inline void ptep_clear(struct mm_struct *mm, unsigned long addr,
>>> -                  pte_t *ptep)
>>> -{
>>> -    pte_clear(mm, addr, ptep);
>>> -}
>>> -#endif
>>> -
>>>   #ifndef __HAVE_ARCH_PTEP_GET_AND_CLEAR
>>>   static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
>>>                          unsigned long address,
>>> @@ -287,10 +280,22 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
>>>   {
>>>       pte_t pte = *ptep;
>>>       pte_clear(mm, address, ptep);
>>> +    page_table_check_pte_clear(mm, address, pte);
>>>       return pte;
>>>   }
>>>   #endif
>>>   +#ifndef __HAVE_ARCH_PTEP_CLEAR
>>> +static inline void ptep_clear(struct mm_struct *mm, unsigned long addr,
>>> +                  pte_t *ptep)
>>> +{
>>> +    if (IS_ENABLED(CONFIG_PAGE_TABLE_CHECK))
>>> +        ptep_get_and_clear(mm, addr, ptep);
>>> +    else
>>> +        pte_clear(mm, addr, ptep);
>>
>> Could not this be reworked to avoid IS_ENABLED() ? This is confusing. If the page
>> table hooks can be added to all potential page table paths via generic helpers,
>> irrespective of CONFIG_PAGE_TABLE_CHECK option, there is no rationale for doing
>> a IS_ENABLED() check here.
>>
> 
> From the perspective of code logic, we need to check the pte before being cleared. Whether pte check is required depends on IS_ENABLED().
> 
> Are there any suggestions for better implementation?

But other generic page table helpers already have page table check hooks
instrumented without IS_ENABLED() checks, then why this is any different.

> 
> Thank you,
> Tong.
> 
>>> +}
>>> +#endif
>>> +
>>>   #ifndef __HAVE_ARCH_PTEP_GET
>>>   static inline pte_t ptep_get(pte_t *ptep)
>>>   {
>>> @@ -360,7 +365,10 @@ static inline pmd_t pmdp_huge_get_and_clear(struct mm_struct *mm,
>>>                           pmd_t *pmdp)
>>>   {
>>>       pmd_t pmd = *pmdp;
>>> +
>>>       pmd_clear(pmdp);
>>> +    page_table_check_pmd_clear(mm, address, pmd);
>>> +
>>>       return pmd;
>>>   }
>>>   #endif /* __HAVE_ARCH_PMDP_HUGE_GET_AND_CLEAR */
>>> @@ -372,6 +380,8 @@ static inline pud_t pudp_huge_get_and_clear(struct mm_struct *mm,
>>>       pud_t pud = *pudp;
>>>         pud_clear(pudp);
>>> +    page_table_check_pud_clear(mm, address, pud);
>>> +
>>>       return pud;
>>>   }
>>>   #endif /* __HAVE_ARCH_PUDP_HUGE_GET_AND_CLEAR */
>> .