Re: [PATCH 0/8] add mTHP support for anonymous shmem

From: Ryan Roberts
Date: Tue May 07 2024 - 06:20:37 EST


On 06/05/2024 09:46, 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.
>
> 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

Didn't we agree that "force" would not be supported for now, and would return an
error when attempting to set for a non-PMD-size hugepage-XXkb/shmem_enabled (or
indirectly through inheritance)?

> '/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(-)
>