[PATCH v3 0/2] mm: improve folio refcount scalability

[ilya.gladyshev@linux.dev]

This is v3 of this series with minor changes since v2 [0]:
- Fix inverted check in second patch
- Rename "unless_zero" functions with "unless_frozen"
- Replace set_page_count(1) with init_page_count()

Small note: We've had to change our email addresses mid-series; apologies
if it confuses anyone. Also sorry for long delay, I will resume my work on
this.

Original cover letter posted below:

Intro
=====
This patch optimizes small file read performance and overall folio refcount
scalability by refactoring page_ref_add_unless [core of folio_try_get].
This is alternative approach to previous attempts to fix small read
performance by avoiding refcount bumps [1][2].

Overview
========
Current refcount implementation is using zero counter as locked (dead/frozen)
state, which required CAS loop for increments to avoid temporary unlocks in
try_get functions. These CAS loops became a serialization point for otherwise
scalable and fast read side.

Proposed implementation separates "locked" logic from the counting, allowing
the use of optimistic fetch_add() instead of CAS. For more details, please
refer to the commit message of the patch itself.

Proposed logic maintains the same public API as before, including all existing
memory barrier guarantees.

Performance
===========
Performance was measured using a simple custom benchmark based on
will-it-scale[3]. This benchmark spawns N pinned threads/processes that
execute the following loop:
``
char buf[]
fd = open(/* same file in tmpfs */);

while (true) {
    pread(fd, buf, /* read size = */ 64, /* offset = */0)
}
``
While this is a synthetic load, it does highlight existing issue and
doesn't differ a lot from benchmarking in [2] patch.

This benchmark measures operations per second in the inner loop and the
results across all workers. Performance was tested on top of v6.15 kernel
on two platforms. Since threads and processes showed similar performance on
both systems, only the thread results are provided below. The performance
improvement scales linearly between the CPU counts shown.

Platform 1: 2 x E5-2690 v3, 12C/12T each [disabled SMT]

#threads | vanilla | patched | boost (%)
       1 | 1343381 | 1344401 |  +0.1
       2 | 2186160 | 2455837 | +12.3
       5 | 5277092 | 6108030 | +15.7
      10 | 5858123 | 7506328 | +28.1
      12 | 6484445 | 8137706 | +25.5
         /* Cross socket NUMA */
      14 | 3145860 | 4247391 | +35.0
      16 | 2350840 | 4262707 | +81.3
      18 | 2378825 | 4121415 | +73.2
      20 | 2438475 | 4683548 | +92.1
      24 | 2325998 | 4529737 | +94.7

Platform 2: 2 x AMD EPYC 9654, 96C/192T each [enabled SMT]

#threads | vanilla | patched | boost (%)
       1 | 1077276 | 1081653 |  +0.4
       5 | 4286838 | 4682513 |  +9.2
      10 | 1698095 | 1902753 | +12.1
      20 | 1662266 | 1921603 | +15.6
      49 | 1486745 | 1828926 | +23.0
      97 | 1617365 | 2052635 | +26.9
         /* Cross socket NUMA */
     105 | 1368319 | 1798862 | +31.5
     136 | 1008071 | 1393055 | +38.2
     168 |  879332 | 1245210 | +41.6
               /* SMT */
     193 |  905432 | 1294833 | +43.0
     289 |  851988 | 1313110 | +54.1
     353 |  771288 | 1347165 | +74.7

[0]: https://lore.kernel.org/lkml/cover.1776350895.git.gorbunov.ivan@h-partners.com/
[1]: https://lore.kernel.org/linux-mm/CAHk-=wj00-nGmXEkxY=-=Z_qP6kiGUziSFvxHJ9N-cLWry5zpA@mail.gmail.com/
[2]: https://lore.kernel.org/linux-mm/20251017141536.577466-1-kirill@shutemov.name/
[3]: https://github.com/antonblanchard/will-it-scale

---

Link to v2: https://lore.kernel.org/lkml/cover.1776350895.git.gorbunov.ivan@h-partners.com/

---
Gladyshev Ilya (1):
  mm: implement page refcount locking via dedicated bit

Gorbunov Ivan (1):
  mm: drop page refcount zero state semantics

 drivers/pci/p2pdma.c               |  4 +--
 include/linux/mm.h                 |  2 +-
 include/linux/page-flags.h         | 13 +++++++
 include/linux/page_ref.h           | 57 ++++++++++++++++++++++++------
 kernel/liveupdate/kexec_handover.c |  6 ++--
 lib/test_hmm.c                     |  4 +--
 mm/hugetlb.c                       |  2 +-
 mm/internal.h                      |  2 +-
 mm/memremap.c                      |  4 +--
 mm/mm_init.c                       |  6 ++--
 mm/page_alloc.c                    |  4 +--
 11 files changed, 77 insertions(+), 27 deletions(-)


base-commit: 6f3ed7fec72fc8979b2a8c7219c0a9fcfc8d07b5
-- 
2.43.0