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

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Date: Tue, 7 May 2024 11:20:24 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 hughd@...gle.com
Cc: willy@...radead.org, david@...hat.com, ioworker0@...il.com,
 wangkefeng.wang@...wei.com, ying.huang@...el.com, 21cnbao@...il.com,
 shy828301@...il.com, ziy@...dia.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/8] add mTHP support for anonymous shmem

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(-)
>