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Subject: Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM
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From:   John Hubbard <jhubbard@nvidia.com>
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Date:   Wed, 4 Nov 2020 10:44:56 -0800
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On 11/4/20 10:17 AM, Jason Gunthorpe wrote:
> 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
> 
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.
  */


thanks,
-- 
John Hubbard
NVIDIA