Re: [PATCH] staging: ion: make the pte default none PTE_RDONLY

[chenfeng <puck.chen@hisilicon.com>]

On 2016/1/16 7:23, Russell King - ARM Linux wrote:
> On Fri, Jan 15, 2016 at 03:03:42PM -0800, Laura Abbott wrote:
>> (adding linux-arm and a few people)
>>
>> On 01/14/2016 06:42 PM, Chen Feng wrote:
>>> The page is already alloc at ion_alloc function,
>>> ion_mmap map the alloced pages to user-space.
>>>
>>> The default prot can be PTE_RDONLY. Take a look at
>>> here:
>>> set_pte_at()
>>> arch/arm64/include/asm:
>>> 		if (pte_dirty(pte) && pte_write(pte))
>>> 			pte_val(pte) &= ~PTE_RDONLY;
>>> 		else
>>> 			pte_val(pte) |= PTE_RDONLY;
>>>
>>> So with the dirty bit,it can improve the efficiency
>>> and donnot need to handle memory fault when use access.
>>>
>>> Signed-off-by: Chen Feng <puck.chen@hisilicon.com>
>>> Signed-off-by: Wei Dong <weidong2@hisilicon.com>
>>> Reviewed-by: Zhuangluan Su <suzhuangluan@hisilicon.com>
>>> ---
>>>  drivers/staging/android/ion/ion.c | 3 +++
>>>  1 file changed, 3 insertions(+)
>>>
>>> diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c
>>> index e237e9f..dba5942 100644
>>> --- a/drivers/staging/android/ion/ion.c
>>> +++ b/drivers/staging/android/ion/ion.c
>>> @@ -1026,6 +1026,9 @@ static int ion_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
>>>  	if (!(buffer->flags & ION_FLAG_CACHED))
>>>  		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
>>>
>>> +	/*Default writeable*/
>>> +	vma->vm_page_prot = pte_mkdirty(vma->vm_page_prot);
>>> +
>>>  	mutex_lock(&buffer->lock);
>>>  	/* now map it to userspace */
>>>  	ret = buffer->heap->ops->map_user(buffer->heap, buffer, vma);
>>>
>>
>> The extra fault is unfortunate but I'm skeptical about just setting
>> pte_mkdirty.
>>
>> Catalin/Will, do you have any thoughts? Right now it seems like any
>> range mapped with remap_pfn_range will have this extra fault
>> behavior. Is marking the range dirty the best solution?
Laura Abbott,

I agree with you, it seems all the remap_pfn_range have this fault behavior.


> 
> What happens if the mapping requested was read only - at the very
> least, I don't think this should be done unconditionally.
> 
Russell,
I am not sure about doing this unconditionally, but it can waste memory&time
while handling page fault with ion alloced page.

And the page can be used directly.