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

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Message-ID: <20201007124409.GN5177@ziepe.ca>
Date:   Wed, 7 Oct 2020 09:44:09 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     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>,
        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 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.

Jason