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

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

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?

What happens if the mapping requested was read only - at the very
least, I don't think this should be done unconditionally.

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