Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM

[Christoph Hellwig]

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.