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    SubjectRe: [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.

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