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

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Message-ID: <20201104164119.GA18218@infradead.org>
Date:   Wed, 4 Nov 2020 16:41:19 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 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.