[PATCH RESEND v2 0/9] Merge arm64/riscv hugetlbfs contpte support
Ryan Roberts
ryan.roberts at arm.com
Mon May 13 01:59:59 PDT 2024
On 12/05/2024 18:25, Alexandre Ghiti wrote:
> 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 :)
Thanks! Yes its unfortunate timing - there are a few topics I would have liked
to get involved with. There's always next year...
>
>> 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?
Yes possibly. One potential future new user of this common code would be arm32
(arch/arm), which also has the contig bit. But AIUI, the pmd an pte formats are
quite different. It's likely that arm would want to opt-in to contpte but not
huge_ptep, so separate selectors may be valuable.
>
>>
>> - 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! If it's too difficult then it's not a deal-breaker. Perhaps just an
initial patch to move the existing arm functions to core-mm without change, then
all your existing patches on top of that would do the job?
>
>>
>> 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
>>>
>>
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