[Question] A question about arm64 pte

[Yisheng Xie]

Hi Catalin,

Thanks again for your reply, and I will check the logic once more.

Thanks
Yisheng Xie

On 2017/1/16 22:36, Catalin Marinas wrote:
> On Mon, Jan 16, 2017 at 08:39:56PM +0800, Yisheng Xie wrote:
>> On 2017/1/16 19:56, Catalin Marinas wrote:
>>> On Mon, Jan 16, 2017 at 06:08:47PM +0800, Yisheng Xie wrote:
>>>> I have question about arm64 pte.
>>>
>>> I assume the context is ARMv8.0 (without hardware DBM support).
>>
>> Yes.
>>>
>>>> For arm64, PTE_WRITE（== PTE_DBM） is to mark whether the page is writable,
>>>> and PTE_DIRTY is to mark whether the page is dirty.
>>>> However, PTE_RDONLY is only cleared when both PTE_WRITE and PTE_DIRTY are set.
>>>
>>> That's what set_pte_at() does.
>>>
>>
>> So if we mmap a memory region use /dev/mem like:
>>    fildes = open("/dev/mem", O_RDWR | O_CREAT, 0777);
>>    addr = mmap(NULL, LEN, PROT_READ | PROT_WRITE, MAP_SHARED, fildes, offset);
>>
>> The PTE_RDONLY will be set， Right ?
> 
> Possibly, I haven't checked mmap_mem(). However, that's what you would
> get with an anonymous mmap() as well.
> 
>> However，when use memset to write the region it still works well, and
>> the bit PTE_RDONLY is also cleared. Is there anywhere clear the
>> PTE_RDONLY before write that page ?
> 
> See handle_pte_fault(). On the first access to a writable+clean page
> (PTE_WRITE set, PTE_RDONLY set, PTE_DIRTY cleared), the kernel traps it
> and, if pte_write() is true (your case), it calls pte_mkdirty(). The
> subsequently called ptep_set_access_flags() function would clear
> PTE_RDONLY, giving you a writable mapping.
>