[PATCH v3 03/13] mm/debug_vm_pgtable/ppc64: Avoid setting top bits in radom value

[Aneesh Kumar K.V]

On 9/1/20 1:16 PM, Anshuman Khandual wrote:
> 
> 
> On 09/01/2020 01:06 PM, Aneesh Kumar K.V wrote:
>> On 9/1/20 1:02 PM, Anshuman Khandual wrote:
>>>
>>>
>>> On 09/01/2020 11:51 AM, Aneesh Kumar K.V wrote:
>>>> On 9/1/20 8:45 AM, Anshuman Khandual wrote:
>>>>>
>>>>>
>>>>> On 08/27/2020 01:34 PM, Aneesh Kumar K.V wrote:
>>>>>> ppc64 use bit 62 to indicate a pte entry (_PAGE_PTE). Avoid setting that bit in
>>>>>> random value.
>>>>>>
>>>>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>
>>>>>> ---
>>>>>>     mm/debug_vm_pgtable.c | 13 ++++++++++---
>>>>>>     1 file changed, 10 insertions(+), 3 deletions(-)
>>>>>>
>>>>>> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
>>>>>> index 086309fb9b6f..bbf9df0e64c6 100644
>>>>>> --- a/mm/debug_vm_pgtable.c
>>>>>> +++ b/mm/debug_vm_pgtable.c
>>>>>> @@ -44,10 +44,17 @@
>>>>>>      * entry type. But these bits might affect the ability to clear entries with
>>>>>>      * pxx_clear() because of how dynamic page table folding works on s390. So
>>>>>>      * while loading up the entries do not change the lower 4 bits. It does not
>>>>>> - * have affect any other platform.
>>>>>> + * have affect any other platform. Also avoid the 62nd bit on ppc64 that is
>>>>>> + * used to mark a pte entry.
>>>>>>      */
>>>>>> -#define S390_MASK_BITS    4
>>>>>> -#define RANDOM_ORVALUE    GENMASK(BITS_PER_LONG - 1, S390_MASK_BITS)
>>>>>> +#define S390_SKIP_MASK        GENMASK(3, 0)
>>>>>> +#ifdef CONFIG_PPC_BOOK3S_64
>>>>>> +#define PPC64_SKIP_MASK        GENMASK(62, 62)
>>>>>> +#else
>>>>>> +#define PPC64_SKIP_MASK        0x0
>>>>>> +#endif
>>>>>
>>>>> Please drop the #ifdef CONFIG_PPC_BOOK3S_64 here. We already accommodate skip
>>>>> bits for a s390 platform requirement and can also do so for ppc64 as well. As
>>>>> mentioned before, please avoid adding any platform specific constructs in the
>>>>> test.
>>>>>
>>>>
>>>>
>>>> that is needed so that it can be built on 32 bit architectures.I did face build errors with arch-linux
>>>
>>> Could not (#if __BITS_PER_LONG == 32) be used instead or something like
>>> that. But should be a generic conditional check identifying 32 bit arch
>>> not anything platform specific.
>>>
>>
>> that _PAGE_PTE bit is pretty much specific to PPC BOOK3S_64.  Not sure why other architectures need to bothered about ignoring bit 62.
> 
> Thats okay as long it does not adversely affect other architectures, ignoring
> some more bits is acceptable. Like existing S390_MASK_BITS gets ignored on all
> other platforms even if it is a s390 specific constraint. Not having platform
> specific #ifdef here, is essential.
> 

Why is it essential?

-aneesh