[PATCH v4 0/2] mm: improve folio refcount scalability

["Gladyshev Ilya" <ilya.gladyshev@linux.dev>]

This is v4 of the series, fixing some dumb mistakes from v3:
- Fix asserts that were never firing
- Rename set_page_count_as_frozen -> set_page_count_frozen
  (
   I don't really like tthe proposed "init" in the function name.
   For consistency, we can rename init_page_count -> set_page_count_init()
   However, if anyone insists, I will use the proposed init_...() naming
  )
- Set proper frozen value in the second patch
- Use VM_BUG_ON_PAGE instead of VM_BUG_ON

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 v3: https://lore.kernel.org/linux-mm/5dabf3a748fee0c7b142c74367e7586f5db1ed1e@linux.dev/

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           | 62 +++++++++++++++++++++++++-----
 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, 82 insertions(+), 27 deletions(-)


base-commit: 2d3090a8aeb596a26935db0955d46c9a5db5c6ce
-- 
2.54.0