Messages in this thread | | | Subject | Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM | From | John Hubbard <> | 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
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