Message-Id: <20231204105440.61448-1-ryan.roberts@arm.com>
Date:   Mon,  4 Dec 2023 10:54:25 +0000
From:   Ryan Roberts <ryan.roberts@....com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Yu Zhao <yuzhao@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        David Hildenbrand <david@...hat.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        John Hubbard <jhubbard@...dia.com>, Zi Yan <ziy@...dia.com>,
        Barry Song <21cnbao@...il.com>,
        Alistair Popple <apopple@...dia.com>,
        Yang Shi <shy828301@...il.com>
Cc:     Ryan Roberts <ryan.roberts@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v3 00/15] Transparent Contiguous PTEs for User Mappings

Hi All,

This is v3 of a series to opportunistically and transparently use contpte
mappings (set the contiguous bit in ptes) for user memory when those mappings
meet the requirements. It is part of a wider effort to improve performance by
allocating and mapping variable-sized blocks of memory (folios). One aim is for
the 4K kernel to approach the performance of the 16K kernel, but without
breaking compatibility and without the associated increase in memory. Another
aim is to benefit the 16K and 64K kernels by enabling 2M THP, since this is the
contpte size for those kernels. We have good performance data that demonstrates
both aims are being met (see below).

Of course this is only one half of the change. We require the mapped physical
memory to be the correct size and alignment for this to actually be useful (i.e.
64K for 4K pages, or 2M for 16K/64K pages). Fortunately folios are solving this
problem for us. Filesystems that support it (XFS, AFS, EROFS, tmpfs, ...) will
allocate large folios up to the PMD size today, and more filesystems are coming.
And the other half of my work, to enable "multi-size THP" (large folios) for
anonymous memory, makes contpte sized folios prevalent for anonymous memory too
[3].

Optimistically, I would really like to get this series merged for v6.8; there is
a chance that the multi-size THP series will also get merged for that version
(although at this point pretty small). But even if it doesn't, this series still
benefits file-backed memory from the file systems that support large folios so
shouldn't be held up for it. Additionally I've got data that shows this series
adds no regression when the system has no appropriate large folios.

All dependecies listed against v1 are now resolved; This series applies cleanly
against v6.7-rc1.

Note that the first two patchs are for core-mm and provides the refactoring to
make some crucial optimizations possible - which are then implemented in patches
14 and 15. The remaining patches are arm64-specific.

Testing
=======

I've tested this series together with multi-size THP [3] on both Ampere Altra
(bare metal) and Apple M2 (VM):
  - mm selftests (inc new tests written for multi-size THP); no regressions
  - Speedometer Java script benchmark in Chromium web browser; no issues
  - Kernel compilation; no issues
  - Various tests under high memory pressure with swap enabled; no issues


Performance
===========

John Hubbard at Nvidia has indicated dramatic 10x performance improvements for
some workloads at [4], when using 64K base page kernel.

You can also see the original performance results I posted against v1 [1] which
are still valid.

I've additionally run the kernel compilation and speedometer benchmarks on a
system with multi-size THP disabled and large folio support for file-backed
memory intentionally disabled; I see no change in performance in this case (i.e.
no regression when this change is "present but not useful").


Changes since v2 [2]
====================

  - Removed contpte_ptep_get_and_clear_full() optimisation for exit() (v2#14),
    and replaced with a batch-clearing approach using a new arch helper,
    clear_ptes() (v3#2 and v3#15) (Alistair and Barry)
  - (v2#1 / v3#1)
      - Fixed folio refcounting so that refcount >= mapcount always (DavidH)
      - Reworked batch demarcation to avoid pte_pgprot() (DavidH)
      - Reverted return semantic of copy_present_page() and instead fix it up in
        copy_present_ptes() (Alistair)
      - Removed page_cont_mapped_vaddr() and replaced with simpler logic
        (Alistair)
      - Made batch accounting clearer in copy_pte_range() (Alistair)
  - (v2#12 / v3#13)
      - Renamed contpte_fold() -> contpte_convert() and hoisted setting/
        clearing CONT_PTE bit to higher level (Alistair)


Changes since v1 [1]
====================

  - Export contpte_* symbols so that modules can continue to call inline
    functions (e.g. ptep_get) which may now call the contpte_* functions (thanks
    to JohnH)
  - Use pte_valid() instead of pte_present() where sensible (thanks to Catalin)
  - Factor out (pte_valid() && pte_cont()) into new pte_valid_cont() helper
    (thanks to Catalin)
  - Fixed bug in contpte_ptep_set_access_flags() where TLBIs were missed (thanks
    to Catalin)
  - Added ARM64_CONTPTE expert Kconfig (enabled by default) (thanks to Anshuman)
  - Simplified contpte_ptep_get_and_clear_full()
  - Improved various code comments


[1] https://lore.kernel.org/linux-arm-kernel/20230622144210.2623299-1-ryan.roberts@arm.com/
[2] https://lore.kernel.org/linux-arm-kernel/20231115163018.1303287-1-ryan.roberts@arm.com/
[3] https://lore.kernel.org/linux-arm-kernel/20231204102027.57185-1-ryan.roberts@arm.com/
[4] https://lore.kernel.org/linux-mm/c507308d-bdd4-5f9e-d4ff-e96e4520be85@nvidia.com/


Thanks,
Ryan

Ryan Roberts (15):
  mm: Batch-copy PTE ranges during fork()
  mm: Batch-clear PTE ranges during zap_pte_range()
  arm64/mm: set_pte(): New layer to manage contig bit
  arm64/mm: set_ptes()/set_pte_at(): New layer to manage contig bit
  arm64/mm: pte_clear(): New layer to manage contig bit
  arm64/mm: ptep_get_and_clear(): New layer to manage contig bit
  arm64/mm: ptep_test_and_clear_young(): New layer to manage contig bit
  arm64/mm: ptep_clear_flush_young(): New layer to manage contig bit
  arm64/mm: ptep_set_wrprotect(): New layer to manage contig bit
  arm64/mm: ptep_set_access_flags(): New layer to manage contig bit
  arm64/mm: ptep_get(): New layer to manage contig bit
  arm64/mm: Split __flush_tlb_range() to elide trailing DSB
  arm64/mm: Wire up PTE_CONT for user mappings
  arm64/mm: Implement ptep_set_wrprotects() to optimize fork()
  arm64/mm: Implement clear_ptes() to optimize exit()

 arch/arm64/Kconfig                |  10 +-
 arch/arm64/include/asm/pgtable.h  | 343 ++++++++++++++++++++---
 arch/arm64/include/asm/tlbflush.h |  13 +-
 arch/arm64/kernel/efi.c           |   4 +-
 arch/arm64/kernel/mte.c           |   2 +-
 arch/arm64/kvm/guest.c            |   2 +-
 arch/arm64/mm/Makefile            |   1 +
 arch/arm64/mm/contpte.c           | 436 ++++++++++++++++++++++++++++++
 arch/arm64/mm/fault.c             |  12 +-
 arch/arm64/mm/fixmap.c            |   4 +-
 arch/arm64/mm/hugetlbpage.c       |  40 +--
 arch/arm64/mm/kasan_init.c        |   6 +-
 arch/arm64/mm/mmu.c               |  16 +-
 arch/arm64/mm/pageattr.c          |   6 +-
 arch/arm64/mm/trans_pgd.c         |   6 +-
 include/asm-generic/tlb.h         |   9 +
 include/linux/pgtable.h           |  39 +++
 mm/memory.c                       | 258 +++++++++++++-----
 mm/mmu_gather.c                   |  14 +
 19 files changed, 1067 insertions(+), 154 deletions(-)
 create mode 100644 arch/arm64/mm/contpte.c

--
2.25.1

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