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

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Message-ID: <20201005183704.GC5177@ziepe.ca>
Date:   Mon, 5 Oct 2020 15:37:04 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     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>,
        Marek Szyprowski <m.szyprowski@...sung.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 Mon, Oct 05, 2020 at 08:16:33PM +0200, Daniel Vetter wrote:

> > kvm is some similar hack added for P2P DMA, see commit
> > add6a0cd1c5ba51b201e1361b05a5df817083618. It might be protected by notifiers..
> 
> Yeah my thinking is that kvm (and I think also vfio, also seems to
> have mmu notifier nearby) are ok because of the mmu notiifer. Assuming
> that one works correctly.

vfio doesn't have a notifier, Alex was looking to add a vfio private
scheme in the vma->private_data:

https://lore.kernel.org/kvm/159017449210.18853.15037950701494323009.stgit@gimli.home/

Guess it never happened.

> > So, the answer really is that s390 and media need fixing, and this API
> > should go away (or become kvm specific)
> 
> I'm still not clear how you want fo fix this, since your vma->dma_buf
> idea is kinda a decade long plan and so just not going to happen:

Well, it doesn't mean we have to change every part of dma_buf to
participate in this. Just the bits media cares about. Or maybe it is
some higher level varient on top of dma_buf.

Or don't use dma_buf for this, add a new object that just provides
refcounts and P2P DMA connection for IO pfn ranges..

Jason