[PATCH RESEND v2 0/9] Merge arm64/riscv hugetlbfs contpte support

[Alexandre Ghiti]

Hi Ryan,

On Fri, May 10, 2024 at 3:49 PM Ryan Roberts <ryan.roberts at arm.com> wrote:
>
> On 08/05/2024 12:34, Alexandre Ghiti wrote:
> > This patchset intends to merge the contiguous ptes hugetlbfs implementation
> > of arm64 and riscv.
> >
> > Both arm64 and riscv support the use of contiguous ptes to map pages that
> > are larger than the default page table size, respectively called contpte
> > and svnapot.
> >
> > The riscv implementation differs from the arm64's in that the LSBs of the
> > pfn of a svnapot pte are used to store the size of the mapping, allowing
> > for future sizes to be added (for now only 64KB is supported). That's an
> > issue for the core mm code which expects to find the *real* pfn a pte points
> > to. Patch 1 fixes that by always returning svnapot ptes with the real pfn
> > and restores the size of the mapping when it is written to a page table.
> >
> > The following patches are just merges of the 2 different implementations
> > that currently exist in arm64 and riscv which are very similar. It paves
> > the way to the reuse of the recent contpte THP work by Ryan [1] to avoid
> > reimplementing the same in riscv.
>
> Hi Alexandre,
>
> I've skimmed through this series and the one that moves contpte. I can see there
> is definitely value in sharing the implementation, and the rough shape of things
> seems appropriate. I had some minor concerns about making it harder to implement
> potential future arm64 errata workarounds but on reflection, most of the
> now-shared code is really just wrapping the primitives that are still arch-specific.
>
> I'm going to need to spend proper time reviewing it to give detailed feedback,
> but I'll be out on paternity leave for 3 weeks from end of Monday at the latest.

Too bad, I expected to discuss that with you at LSF/MM...But congrats!
Hope your wife is fine :)

> So realistically I won't be able to do the detailed review until at least the
> first week of June.
>
> Some high level thoughts:
>
>  - huge_ptep_* functions could be working on different sized huge ptes - arm64
> supports contpte, pmd, contpmd and pud. Is keeping them in contpte.c
> appropriate?

Hmm indeed, I'll see what I can do.

> Perhaps it's better to keep huge_pte and contpte separate? Also, it
> only works on arm64 because we can get away with calling the lower-level pte
> functions even when the huge_pte is actually a contpmd/pmd/pud, because the
> format is the same. That might present challenges to other arches if the format
> is different?

Yes, but I think that if that happens, we could get away with it by
choosing the right function depending on the size of the mapping?

>
>  - It might be easier to review if the arm64 stuff is first moved (without
> changes) then modified to make it suitable for riscv, then for riscv to be
> hooked up. At the moment I'm trying to follow all 3 parts per-function.

Ok, let me give it a try during your paternity leave!

>
> Thanks,
> Ryan

Thanks,

Alex

>
>
> >
> > This patchset was tested by running the libhugetlbfs testsuite with 64KB
> > and 2MB pages on both architectures (on a 4KB base page size arm64 kernel).
> >
> > [1] https://lore.kernel.org/linux-arm-kernel/20240215103205.2607016-1-ryan.roberts@arm.com/
> >
> > Changes in v2:
> >   - Rebase on top of 6.9-rc3
> >
> > Alexandre Ghiti (9):
> >   riscv: Restore the pfn in a NAPOT pte when manipulated by core mm code
> >   riscv: Safely remove huge_pte_offset() when manipulating NAPOT ptes
> >   mm: Use common huge_ptep_get() function for riscv/arm64
> >   mm: Use common set_huge_pte_at() function for riscv/arm64
> >   mm: Use common huge_pte_clear() function for riscv/arm64
> >   mm: Use common huge_ptep_get_and_clear() function for riscv/arm64
> >   mm: Use common huge_ptep_set_access_flags() function for riscv/arm64
> >   mm: Use common huge_ptep_set_wrprotect() function for riscv/arm64
> >   mm: Use common huge_ptep_clear_flush() function for riscv/arm64
> >
> >  arch/arm64/Kconfig                  |   1 +
> >  arch/arm64/include/asm/pgtable.h    |  56 +++++-
> >  arch/arm64/mm/hugetlbpage.c         | 291 +---------------------------
> >  arch/riscv/Kconfig                  |   1 +
> >  arch/riscv/include/asm/hugetlb.h    |   2 +-
> >  arch/riscv/include/asm/pgtable-64.h |  11 ++
> >  arch/riscv/include/asm/pgtable.h    | 153 +++++++++++++--
> >  arch/riscv/mm/hugetlbpage.c         | 227 ----------------------
> >  arch/riscv/mm/pgtable.c             |   6 +-
> >  mm/Kconfig                          |   3 +
> >  mm/Makefile                         |   1 +
> >  mm/contpte.c                        | 272 ++++++++++++++++++++++++++
> >  12 files changed, 480 insertions(+), 544 deletions(-)
> >  create mode 100644 mm/contpte.c
> >
>