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

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Message-ID: <CAKMK7uG3fds79Yf9VhMstnJ2+UHYUEVdODkoOvtwFC28_+T6RA@mail.gmail.com>
Date:   Wed, 7 Oct 2020 14:58:33 +0200
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        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>,
        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 2:48 PM Tomasz Figa <tfiga@...omium.org> wrote:
>
> On Wed, Oct 7, 2020 at 2:44 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
> >
> > On Wed, Oct 07, 2020 at 02:33:56PM +0200, Marek Szyprowski wrote:
> > > Well, it was in vb2_get_vma() function, but now I see that it has been
> > > lost in fb639eb39154 and 6690c8c78c74 some time ago...
> >
> > There is no guarentee that holding a get on the file says anthing
> > about the VMA. This needed to check that the file was some special
> > kind of file that promised the VMA layout and file lifetime are
> > connected.
> >
> > Also, cloning a VMA outside the mm world is just really bad. That
> > would screw up many assumptions the drivers make.
> >
> > If it is all obsolete I say we hide it behind a default n config
> > symbol and taint the kernel if anything uses it.
> >
> > Add a big comment above the follow_pfn to warn others away from this
> > code.
>
> Sadly it's just verbally declared as deprecated and not formally noted
> anyway. There are a lot of userspace applications relying on user
> pointer support.

userptr can stay, it's the userptr abuse for zerocpy buffer sharing
which doesn't work anymore. At least without major surgery (you'd need
an mmu notifier to zap mappings and recreate them, and that pretty
much breaks the v4l model of preallocating all buffers to make sure we
never underflow the buffer queue). And static mappings are not coming
back I think, we'll go ever more into the direction of dynamic
mappings and moving stuff around as needed.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch