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

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 <redacted>
Signed-off-by: Wei Dong <redacted>
Reviewed-by: Zhuangluan Su <redacted>
---
  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);