On Wed, Nov 04, 2020 at 10:44:56AM -0800, John Hubbard wrote:

On 11/4/20 10:17 AM, Jason Gunthorpe wrote:

quoted

On Wed, Nov 04, 2020 at 04:41:19PM +0000, Christoph Hellwig wrote:

quoted

On Wed, Nov 04, 2020 at 04:37:58PM +0000, Christoph Hellwig wrote:

quoted

On Wed, Nov 04, 2020 at 05:26:58PM +0100, Daniel Vetter wrote:

quoted

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

Well, the !CONFIG_ARCH_HAS_PTE_SPECIAL case points out in a comment that
gup-fast is not *completely* unavailable there, so I don't think you want
to shut it off like that:

/*
 * If we can't determine whether or not a pte is special, then fail immediately
 * for ptes. Note, we can still pin HugeTLB and THP as these are guaranteed not
 * to be special.
 *
 * For a futex to be placed on a THP tail page, get_futex_key requires a
 * get_user_pages_fast_only implementation that can pin pages. Thus it's still
 * useful to have gup_huge_pmd even if we can't operate on ptes.
 */

I saw that once and I really couldn't make sense of it..
What use is having futex's that only work on THP pages? Confused

CH said there was no case of HAVE_FAST_GUP !ARCH_HAS_PTE_SPECIAL, is
one hidden someplace then?

Jason

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