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

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Date:   Wed, 4 Nov 2020 17:26:58 +0100
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        "J??r??me Glisse" <jglisse@...hat.com>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, Pawel Osciak <pawel@...iak.com>,
        KVM list <kvm@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Linux MM <linux-mm@...ck.org>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM

On Wed, Nov 4, 2020 at 5:21 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Wed, Nov 04, 2020 at 04:54:19PM +0100, Daniel Vetter wrote:
> > 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.
>
> dma_mmap_* memory may or may not be page backed, but it absolutely
> must not be resolved by get_user_pages and friends as it is special.
> So yes, not being able to get a struct page back from such an mmap is
> a feature.

Yes, that's clear.

What we're discussing is whether gup_fast and pup_fast also obey this,
or fall over and can give you the struct page that's backing the
dma_mmap_* memory. Since the _fast variant doesn't check for
vma->vm_flags, and afaict that's the only thing which closes this gap.
And like you restate, that would be a bit a problem. So where's that
check which Jason&me aren't spotting?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch