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

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Message-ID: <CAKMK7uF+a1PSn+e-6F+YhkSXn9vC7etS-z0AFBMCU+Vzb2PwqA@mail.gmail.com>
Date:   Wed, 7 Oct 2020 14:01:44 +0200
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
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

On Wed, Oct 7, 2020 at 12:47 PM Marek Szyprowski
<m.szyprowski@...sung.com> wrote:
>
> Hi Daniel,
>
> 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?
-Daniel

> This API is considered as deprecated in V4L2 world, so I think
> supporting this hack can be removed one day as nowadays userspace should
> use dma buf.
>
>  > ...
>
> Best regards
> --
> Marek Szyprowski, PhD
> Samsung R&D Institute Poland
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch