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Date:   Wed, 4 Nov 2020 14:17:08 -0400
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Daniel Vetter <daniel.vetter@ffwll.ch>,
        J??r??me Glisse <jglisse@redhat.com>,
        linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
        Jan Kara <jack@suse.cz>, Pawel Osciak <pawel@osciak.com>,
        KVM list <kvm@vger.kernel.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Tomasz Figa <tfiga@chromium.org>,
        Linux MM <linux-mm@kvack.org>,
        Kyungmin Park <kyungmin.park@samsung.com>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>
Subject: Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM
Message-ID: <20201104181708.GU36674@ziepe.ca>
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On Wed, Nov 04, 2020 at 04:41:19PM +0000, Christoph Hellwig wrote:
> On Wed, Nov 04, 2020 at 04:37:58PM +0000, Christoph Hellwig wrote:
> > On Wed, Nov 04, 2020 at 05:26:58PM +0100, Daniel Vetter wrote:
> > > 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?
> > 
> > remap_pte_range uses pte_mkspecial to set up the PTEs, and gup_pte_range
> > errors out on pte_special.  Of course this only works for the
> > CONFIG_ARCH_HAS_PTE_SPECIAL case, for other architectures we do have
> > a real problem.
> 
> Except that we don't really support pte-level gup-fast without
> CONFIG_ARCH_HAS_PTE_SPECIAL, and in fact all architectures selecting
> HAVE_FAST_GUP also select ARCH_HAS_PTE_SPECIAL, so we should be fine.

Mm, I thought it was probably the special flag..

Knowing that CONFIG_HAVE_FAST_GUP can't be set without
CONFIG_ARCH_HAS_PTE_SPECIAL is pretty insightful, can we put that in
the Kconfig?

config HAVE_FAST_GUP
        depends on MMU
        depends on ARCH_HAS_PTE_SPECIAL
        bool

?

Jason