linux-kernel - Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL

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Date:   Wed, 4 Nov 2020 16:54:19 +0100
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     John Hubbard <jhubbard@...dia.com>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Pawel Osciak <pawel@...iak.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM

On Wed, Nov 4, 2020 at 3:00 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Sun, Nov 01, 2020 at 11:50:39PM +0100, Daniel Vetter wrote:
>
> > It's not device drivers, but everyone else. At least my understanding
> > is that VM_IO | VM_PFNMAP means "even if it happens to be backed by a
> > struct page, do not treat it like normal memory". And gup/pup_fast
> > happily break that. I tried to chase the history of that test, didn't
> > turn up anything I understood much:
>
> VM_IO isn't suppose do thave struct pages, so how can gup_fast return
> them?
>
> I thought some magic in the PTE flags excluded this?

I don't really have a box here, but dma_mmap_attrs() and friends to
mmap dma_alloc_coherent memory is set up as VM_IO | VM_PFNMAP (it's
actually enforced since underneath it uses remap_pfn_range), and
usually (except if it's pre-cma carveout) that's just normal struct
page backed memory. Sometimes from a cma region (so will be caught by
the cma page check), but if you have an iommu to make it
device-contiguous, that's not needed.

I think only some architectures have a special io pte flag, and those
are only used for real mmio access. And I think the popular ones all
don't. But that stuff is really not my expertise, just some drive-by
reading I've done to understand how the pci mmap stuff works (which is
special in yet other ways I think).

So probably I'm missing something, but I'm not seeing anything that
prevents this from coming out of a  pup/gup_fast.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch