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

[Jason Gunthorpe]

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