linux-kernel - Re: [PATCH 2/2] mm/frame-vec: use FOLL

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Date:   Wed, 7 Oct 2020 14:33:56 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Pawel Osciak <pawel@...iak.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Inki Dae <inki.dae@...sung.com>,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>, Oded Gabbay <oded.gabbay@...il.com>
Subject: Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM

Hi Daniel,

On 07.10.2020 14:01, Daniel Vetter wrote:
> On Wed, Oct 7, 2020 at 12:47 PM Marek Szyprowski
> <m.szyprowski@...sung.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