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

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Message-ID: <CAAFQd5D0ahP-3hp_vGEmJ2cyBOMXeW9HX5yKaVPcQTsFwwOE8Q@mail.gmail.com>
Date:   Wed, 7 Oct 2020 14:47:55 +0200
From:   Tomasz Figa <tfiga@...omium.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        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 2:44 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.