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

Daniel Vetter
2020-11-06T10:02:16Z

On Fri, Nov 6, 2020 at 5:08 AM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 11/5/20 4:49 AM, Jason Gunthorpe wrote:
> > On Thu, Nov 05, 2020 at 10:25:24AM +0100, Daniel Vetter wrote:
> >>> /*
> >>> * 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.
> >
> > It means ioremap can't create an IO page PUD, it has to be broken up.
> >
> > Does ioremap even create anything larger than PTEs?

gpu drivers also tend to use vmf_insert_pfn* directly, so we can do
on-demand paging and move buffers around. From what I glanced for
lowest level we to the pte_mkspecial correctly (I think I convinced
myself that vm_insert_pfn does that), but for pud/pmd levels it seems
just yolo.

remap_pfn_range seems to indeed split down to pte level always.

> From my reading, yes. See ioremap_try_huge_pmd().

The ioremap here shouldn't matter, since this is for kernel-internal
mappings. So that's all fine I think.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation