[RFC PATCH v1 00/17] ban the use of _PAGE_XXX flags outside platform specific code

[Christophe LEROY]

Le 10/09/2018 à 08:08, Aneesh Kumar K.V a écrit :
> Christophe Leroy <christophe.leroy at c-s.fr> writes:
> 
>> On 09/06/2018 09:58 AM, Aneesh Kumar K.V wrote:
>>> Christophe Leroy <christophe.leroy at c-s.fr> writes:
>>>
>>>> Today flags like for instance _PAGE_RW or _PAGE_USER are used through
>>>> common parts of code.
>>>> Using those directly in common parts of code have proven to lead to
>>>> mistakes or misbehaviour, because their use is not always as trivial
>>>> as one could think.
>>>>
>>>> For instance, (flags & _PAGE_USER) == 0 isn't enough to tell
>>>> that a page is a kernel page, because some targets are using
>>>> _PAGE_PRIVILEDGED and not _PAGE_USER, so the test has to be
>>>> (flags & (_PAGE_USER | _PAGE_PRIVILEDGED)) == _PAGE_PRIVILEDGED
>>>> This has to (bad) consequences:
>>>>
>>>>    - All targets must define every bit, even the unsupported ones,
>>>>      leading to a lot of useless #define _PAGE_XXX 0
>>>>    - If someone forgets to take into account all possible _PAGE_XXX bits
>>>>      for the case, we can get unexpected behaviour on some targets.
>>>>
>>>> This becomes even more complex when we come to using _PAGE_RW.
>>>> Testing (flags & _PAGE_RW) is not enough to test whether a page
>>>> if writable or not, because:
>>>>
>>>>    - Some targets have _PAGE_RO instead, which has to be unset to tell
>>>>      a page is writable
>>>>    - Some targets have _PAGE_R and _PAGE_W, in which case
>>>>      _PAGE_RW = _PAGE_R | _PAGE_W
>>>>    - Even knowing whether a page is readable is not always trivial because:
>>>>      - Some targets requires to check that _PAGE_R is set to ensure page
>>>>      is readable
>>>>      - Some targets requires to check that _PAGE_NA is not set
>>>>      - Some targets requires to check that _PAGE_RO or _PAGE_RW is set
>>>>
>>>> Etc ....
>>>>
>>>> In order to work around all those issues and minimise the risks of errors,
>>>> this serie aims at removing all use of _PAGE_XXX flags from powerpc code
>>>> and always use pte_xxx() and pte_mkxxx() accessors instead. Those accessors
>>>> are then defined in target specific parts of the kernel code.
>>>
>>> The series is really good. It also helps in code readability. Few things
>>> i am not sure there is a way to reduce the overhead
>>>
>>> -		access = _PAGE_EXEC;
>>> +		access = pte_val(pte_mkexec(__pte(0)));
>>>
>>> Considering we have multiple big endian to little endian coversion there
>>> for book3s 64.
>>
>> Thanks for the review.
>>
>> For the above, I propose the following:
>>
>> diff --git a/arch/powerpc/mm/hash_utils_64.c
>> b/arch/powerpc/mm/hash_utils_64.c
>> index f23a89d8e4ce..904ac9c84ea5 100644
>> --- a/arch/powerpc/mm/hash_utils_64.c
>> +++ b/arch/powerpc/mm/hash_utils_64.c
>> @@ -1482,7 +1482,7 @@ static bool should_hash_preload(struct mm_struct
>> *mm, unsigned long ea)
>>    #endif
>>
>>    void hash_preload(struct mm_struct *mm, unsigned long ea,
>> -		  unsigned long access, unsigned long trap)
>> +		  bool is_exec, unsigned long trap)
>>    {
>>    	int hugepage_shift;
>>    	unsigned long vsid;
>> @@ -1490,6 +1490,7 @@ void hash_preload(struct mm_struct *mm, unsigned
>> long ea,
>>    	pte_t *ptep;
>>    	unsigned long flags;
>>    	int rc, ssize, update_flags = 0;
>> +	unsigned long access = is_exec ? _PAGE_EXEC : 0;
> 
> 
> I guess it will be better if we do
> 
> unsigned long access = _PAGE_PRESENT | _PAGE_READ
> 
> if (is_exec)
>     access |= _PAGE_EXEC.
> 
> That will also bring it closer to __hash_page. I agree that we should
> always find _PAGE_PRESENT and _PAGE_READ set, because we just handled
> the page fault.
> 

Ok, I did it in v2, can you have a look (patch 11/24) ?

Christophe