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    SubjectRe: [RFC PATCH 0/5] add mTHP support for anonymous share pages
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    On 2024/4/23 18:41, Ryan Roberts wrote:
    > On 22/04/2024 08:02, Baolin Wang wrote:
    >> 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.
    >
    > This sounds like a very useful addition!
    >
    > Out of interest, can you point me at any workloads (and off-the-shelf benchmarks
    > for those workloads) that predominantly use shared anon memory?

    As far as I know, some database related workloads make extensive use of
    shared anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL
    likely also uses shared anonymous memory. And I still need to do some
    investigation to measure the performance.

    [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL

    >> The primary strategy is that, the use of huge pages for anonymous shared pages
    >> still follows the global control determined by the mount option "huge=" parameter
    >> or the sysfs interface at '/sys/kernel/mm/transparent_hugepage/shmem_enabled'.
    >> The utilization of mTHP is allowed only when the global 'huge' switch is enabled.
    >> Subsequently, the mTHP sysfs interface (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled)
    >> is checked to determine the mTHP size that can be used for large folio allocation
    >> for these anonymous shared pages.
    >
    > I'm not sure about this proposed control mechanism; won't it break
    > compatibility? I could be wrong, but I don't think shmem's use of THP used to
    > depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it

    Yes, I realized this after more testing.

    > doesn't make sense to me that we now depend upon the
    > /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by
    > default disables all sizes except 2M, which is set to "inherit" from
    > /sys/kernel/mm/transparent_hugepage/enabled).
    >
    > The other problem is that shmem_enabled has a different set of options
    > (always/never/within_size/advise/deny/force) to enabled (always/madvise/never)
    >
    > Perhaps it would be cleaner to do the same trick we did for enabled; Introduce
    > /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,
    > plus the additional "inherit" option. By default all sizes will be set to
    > "never" except 2M, which is set to "inherit".

    Sounds good to me. But I do not want to copy all same values from
    top-level '/sys/kernel/mm/transparent_hugepage/shmem_enabled':
    always within_size advise never deny force

    For mTHP's shmem_enabled interface, we can just keep below values:
    always within_size advise never

    Cause when checking if mTHP can be used for anon shmem, 'deny' is equal
    to 'never', and 'force' is equal to 'always'.

    > Of course the huge= mount option would also need to take a per-size option in
    > this case. e.g. huge=2048kB:advise,64kB:always

    IMO, I do not want to change the global 'huge=' mount option, which can
    control both anon shmem and tmpfs, but mTHP now is only applied for anon
    shmem. So let's keep it be same with the global sysfs interface:
    /sys/kernel/mm/transparent_hugepage/shmem_enabled.

    For tmpfs large folio strategy, I plan to address it later, and we may
    need more discussion to determine if it should follow the file large
    folio strategy or not (no investigation now).

    Thanks for reviewing.

    >> TODO:
    >> - More testing and provide some performance data.
    >> - Need more discussion about the large folio allocation strategy for a 'regular
    >> file' operation created by memfd_create(), for example using ftruncate(fd) to specify
    >> the 'file' size, which need to follow the anonymous mTHP rule too?
    >> - Do not split the large folio when share memory swap out.
    >> - Can swap in a large folio for share memory.
    >>
    >> Baolin Wang (5):
    >> 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 mTHP support for anonymous share pages
    >> mm: shmem: add anonymous share mTHP counters
    >>
    >> include/linux/huge_mm.h | 4 +-
    >> mm/huge_memory.c | 8 ++-
    >> mm/memory.c | 25 +++++++---
    >> mm/shmem.c | 107 ++++++++++++++++++++++++++++++----------
    >> 4 files changed, 108 insertions(+), 36 deletions(-)
    >>

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