On Wed, Nov 04, 2020 at 10:44:56AM -0800, John Hubbard wrote: > 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. Thanks for the explainer. I guess I can go back to _fast and instead adjust the commit message to explain why that's all 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. > */ We support hugepage faults in gpu drivers since recently, and I'm not seeing a pud_mkhugespecial anywhere. So not sure this works, but probably just me missing something again. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
