Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hi Daniel,

On 07.10.2020 14:01, Daniel Vetter wrote:
> On Wed, Oct 7, 2020 at 12:47 PM Marek Szyprowski
> <m.szyprowski@samsung.com> wrote:
>> On 03.10.2020 11:40, Daniel Vetter wrote:
>>>> After he three places above should use pin_user_pages_fast(), then
>>>> this whole broken API should be moved into videobuf2-memops.c and a
>>>> big fat "THIS DOESN'T WORK" stuck on it.
>>>>
>>>> videobuf2 should probably use P2P DMA buf for this instead.
>>> Yup this should be done with dma_buf instead, and v4l has that.
>> Yes, V4L2 has dma_buf support NOW. That days, using so called V4L2
>> USERPTR method was the only way to achieve zero copy buffer sharing
>> between devices, so this is just a historical baggage. I've been
>> actively involved in implementing that. I've tried to make it secure as
>> much as possible assuming the limitation of that approach. With a few
>> assumptions it works fine. Buffers are refcounted both by the
>> vm_ops->open or by incrementing the refcount of the vm->file. This
>> basically works with any sane driver, which doesn't free the mmaped
>> buffer until the file is released. This is true for V4L2 and FBdev devices.
> I'm not seeing any of that vm->file refcounting going on, so not
> seeing anything that prevents the mmap area from being removed. Can
> you pls give me some pointers about which code you're thinking of?

Well, it was in vb2_get_vma() function, but now I see that it has been 
lost in fb639eb39154 and 6690c8c78c74 some time ago...

Best regards

-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland