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

On 2016/1/16 7:23, Russell King - ARM Linux wrote:

On Fri, Jan 15, 2016 at 03:03:42PM -0800, Laura Abbott wrote:

quoted

(adding linux-arm and a few people)

On 01/14/2016 06:42 PM, Chen Feng wrote:

quoted

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);

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.