linux-kernel - Re: [PATCH 0/9] Merge arm64/riscv hugetlbfs contpte support

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Message-ID: <mhng-911ba065-e6c8-4d42-978c-e47897bcb493@palmer-ri-x1c9a>
Date: Thu, 18 Apr 2024 15:11:56 -0700 (PDT)
From: Palmer Dabbelt <palmer@...belt.com>
To: alexghiti@...osinc.com
CC: ryan.roberts@....com, Catalin Marinas <catalin.marinas@....com>,
  Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>, Paul Walmsley <paul.walmsley@...ive.com>,
  aou@...s.berkeley.edu, akpm@...ux-foundation.org, linux-arm-kernel@...ts.infradead.org,
  linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org, linux-mm@...ck.org
Subject:     Re: [PATCH 0/9] Merge arm64/riscv hugetlbfs contpte support

On Fri, 01 Mar 2024 03:29:18 PST (-0800), alexghiti@...osinc.com wrote:
> Hi Ryan,
>
> On Fri, Mar 1, 2024 at 11:45 AM Ryan Roberts <ryan.roberts@....com> wrote:
>>
>> Hi Alexandre,
>>
>> I confess I haven't looked at the patches yet, but this cover letter raises a
>> few quesions for me. I'll aim to look at the actual patches in due course.

Acked-by: Palmer Dabbelt <palmer@...osinc.com>

in case someone wants to pick them up via a generic tree.  I'm happy to 
take them via the RISC-V tree if folk want, no rush on my end I'm just 
scrubbing through old stuff.

>> On 01/03/2024 09:14, 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.
>>
>> Yes that makes sense to me. The intention for mTHP (!hugetlb) is to fully
>> encapsulate PTEs beind set_ptes(), ptep_get() and friends, so what's actually
>> written to the pgtable is arch-specific and well abstracted.
>>
>> >
>> > 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.
>>
>> You seem to be talking about both hugetlb (which uses the "huge" pte helpers)
>> and contpte for THP (i.e. mTHP, which uses the regular pte helpers). They are
>> pretty separate in my mind, so not sure why you would be modifying them both in
>> the same series?
>
> I don't, this patchset only deals with hugetlb, I just meant that this
> series was just the beginning as I'm working on moving the contpte for
> THP support in the generic code for riscv to use.
>
> Sorry my wording was ambiguous :)
>
> Thanks,
>
> Alex
>
>>
>> Thanks,
>> Ryan
>>
>> >
>> > 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/
>> >
>> > 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    |  59 +++++-
>> >  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    | 120 +++++++++++-
>> >  arch/riscv/mm/hugetlbpage.c         | 227 ----------------------
>> >  mm/Kconfig                          |   3 +
>> >  mm/Makefile                         |   1 +
>> >  mm/contpte.c                        | 268 +++++++++++++++++++++++++
>> >  11 files changed, 456 insertions(+), 528 deletions(-)
>> >  create mode 100644 mm/contpte.c
>> >
>>