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

[Ryan Roberts]

On 01/03/2024 11:29, Alexandre Ghiti wrote:
> Hi Ryan,
> 
> On Fri, Mar 1, 2024 at 11:45 AM Ryan Roberts <ryan.roberts@xxxxxxx> 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.
>>
>> 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.

Ahh got it! Thanks for the explanation.

> 
> 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@xxxxxxx/
>>>
>>> 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
>>>
>>