Re: [PATCH v4 4/9] drm/rockchip/rockchip_drm_gem.c: Convert to use vm_insert_range

On Tue, Dec 18, 2018 at 06:24:29PM +0530, Souptick Joarder wrote:

On Tue, Dec 18, 2018 at 6:03 PM Russell King - ARM Linux
[off-list ref] wrote:

quoted

On Tue, Dec 18, 2018 at 05:36:04PM +0530, Souptick Joarder wrote:

quoted

On Tue, Dec 18, 2018 at 3:27 PM Russell King - ARM Linux
[off-list ref] wrote:

quoted

This looks like a change in behaviour.

If user_count is zero, and offset is zero, then we pass into
vm_insert_range() a page_count of zero, and vm_insert_range() does
nothing and returns zero.

However, as we can see from the above code, the original behaviour
was to return -ENXIO in that case.

I think these checks are not necessary. I am not sure if we get into mmap
handlers of driver with user_count = 0.

I'm not sure either, I'm just pointing out the change of behaviour.

Ok. I think feedback from Heiko might be helpful here :)

quoted

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The other thing that I'm wondering is that if (eg) count is 8 (the
object is 8 pages), offset is 2, and the user requests mapping 6
pages (user_count = 6), then we call vm_insert_range() with a
pages of rk_obj->pages + 2, and a pages_count of 6 - 2 = 4. So we
end up inserting four pages.

Considering the scenario, user_count will remain 8 (user_count =
vma_pages(vma) ). ? No ?
Then we call vm_insert_range() with a pages of rk_obj->pages + 2, and
a pages_count
of 8 - 2 = 6. So we end up inserting 6 pages.

Please correct me if I am wrong.

vma_pages(vma) is the number of pages that the user requested, it is
the difference between vma->vm_end and vma->vm_start in pages.  As I
said above, "the user requests mapping 6 pages", so vma_pages() will
be 6, and so user_count will also be 6.  You are passing
user_count - offset into vm_insert_range(), which will be 6 - 2 = 4
in my example.  This is two pages short of what the user requested.

So, this should be the correct behavior.

                 return vm_insert_range(vma, vma->vm_start,
rk_obj->pages + offset,
                                                          user_count);

... and by doing so, you're introducing another instance of the same
bug I pointed out in patch 2.

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