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

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Message-ID: <20201007134234.GR5177@ziepe.ca>
Date:   Wed, 7 Oct 2020 10:42:34 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        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 07, 2020 at 03:34:01PM +0200, Tomasz Figa wrote:

> I think the userptr zero-copy hack should be able to go away indeed,
> given that we now have CMA that allows having carveouts backed by
> struct pages and having the memory represented as DMA-buf normally.

This also needs to figure out how to get references to CMA pages out
of a VMA. IIRC Daniel said these are not pinnable?

> How about the regular userptr use case, though?

Just call pin_user_pages(), that is the easy case.

> Is your intention to drop get_vaddr_frames() or we could still keep
> using it and if vec->is_pfns is true:

get_vaddr_frames() is dangerous, I would like it to go away.

> a) if CONFIG_VIDEO_LEGACY_PFN_USERPTR is set, taint the kernel
> b) otherwise just undo and fail?

For the CONFIG_VIDEO_LEGACY_PFN_USERPTR case all the follow_pfn
related code in get_vaddr_frames() shold move back into media and be
hidden under this config.

Jason