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    Subject[PATCH 0/8] add mTHP support for anonymous shmem
    Date
    Anonymous pages have already been supported for multi-size (mTHP) allocation
    through commit 19eaf44954df, that can allow THP to be configured through the
    sysfs interface located at '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'.

    However, the anonymous shared pages will ignore the anonymous mTHP rule
    configured through the sysfs interface, and can only use the PMD-mapped
    THP, that is not reasonable. Many implement anonymous page sharing through
    mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios,
    therefore, users expect to apply an unified mTHP strategy for anonymous pages,
    also including the anonymous shared pages, in order to enjoy the benefits of
    mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat
    than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss etc.

    The primary strategy is similar to supporting anonymous mTHP. Introduce
    a new interface '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled',
    which can have all the same values as the top-level
    '/sys/kernel/mm/transparent_hugepage/shmem_enabled', with adding a new
    additional "inherit" option. By default all sizes will be set to "never"
    except PMD size, which is set to "inherit". This ensures backward compatibility
    with the shmem enabled of the top level, meanwhile also allows independent
    control of shmem enabled for each mTHP.

    Use the page fault latency tool to measure the performance of 1G anonymous shmem
    with 32 threads on my machine environment with: ARM64 Architecture, 32 cores,
    125G memory:
    base: mm-unstable
    user-time sys_time faults_per_sec_per_cpu faults_per_sec
    0.04s 3.10s 83516.416 2669684.890

    mm-unstable + patchset, anon shmem mTHP disabled
    user-time sys_time faults_per_sec_per_cpu faults_per_sec
    0.02s 3.14s 82936.359 2630746.027

    mm-unstable + patchset, anon shmem 64K mTHP enabled
    user-time sys_time faults_per_sec_per_cpu faults_per_sec
    0.08s 0.31s 678630.231 17082522.495

    From the data above, it is observed that the patchset has a minimal impact when
    mTHP is not enabled (some fluctuations observed during testing). When enabling 64K
    mTHP, there is a significant improvement of the page fault latency.

    TODO:
    - Support mTHP for tmpfs.
    - Do not split the large folio when share memory swap out.
    - Can swap in a large folio for share memory.

    Changes from RFC:
    - Rebase the patch set against the new mm-unstable branch, per Lance.
    - Add a new patch to export highest_order() and next_order().
    - Add a new patch to align mTHP size in shmem_get_unmapped_area().
    - Handle the uffd case and the VMA limits case when building mapping for
    large folio in the finish_fault() function, per Ryan.
    - Remove unnecessary 'order' variable in patch 3, per Kefeng.
    - Keep the anon shmem counters' name consistency.
    - Modify the strategy to support mTHP for anonymous shmem, discussed with
    Ryan and David.
    - Add reviewed tag from Barry.
    - Update the commit message.

    Baolin Wang (8):
    mm: move highest_order() and next_order() out of the THP config
    mm: memory: extend finish_fault() to support large folio
    mm: shmem: add an 'order' parameter for shmem_alloc_hugefolio()
    mm: shmem: add THP validation for PMD-mapped THP related statistics
    mm: shmem: add multi-size THP sysfs interface for anonymous shmem
    mm: shmem: add mTHP support for anonymous shmem
    mm: shmem: add mTHP size alignment in shmem_get_unmapped_area
    mm: shmem: add mTHP counters for anonymous shmem

    Documentation/admin-guide/mm/transhuge.rst | 29 ++
    include/linux/huge_mm.h | 35 ++-
    mm/huge_memory.c | 17 +-
    mm/memory.c | 43 ++-
    mm/shmem.c | 335 ++++++++++++++++++---
    5 files changed, 387 insertions(+), 72 deletions(-)

    --
    2.39.3


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