Re: [PATCH] staging: ion: make the pte default none PTE_RDONLY
Date: Sun Jan 17 2016 - 22:45:41 EST
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
>>> if (pte_dirty(pte) && pte_write(pte))
>>> pte_val(pte) &= ~PTE_RDONLY;
>>> 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@xxxxxxxxxxxxx>
>>> Signed-off-by: Wei Dong <weidong2@xxxxxxxxxxxxx>
>>> Reviewed-by: Zhuangluan Su <suzhuangluan@xxxxxxxxxxxxx>
>>> 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);
>>> /* 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
>> 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?
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.
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.