[PATCH RFC 0/3] mm/ksm: scan with per-VMA locks

[Longlong Xia]

From: Longlong Xia <xialonglong@xxxxxxxxxx>

This work was developed independently before I saw the above series.
After comparing the two implementations, I found that the approaches
overlap, so I am posting this draft version as an alternative 
implementation and would be happy to combine the useful parts.

  The main differences are:

  - use per-VMA locking in the ksmd VMA scanner;
  - revalidate the mmap-lock sequence before skipping the fallback walk;
  - avoid repeating a completed VMA traversal when no candidate page is
    found.

Benchmark (QEMU q35, TCG multi-thread, 4 vCPUs, 8 GiB, x86_64 guest on
Ubuntu 24.04, 7.3-rc2 base, CONFIG_KSM=y + CONFIG_PER_VMA_LOCK=y;
victim: 32 MiB anon, 2048 unique pages x4 duplicates,
pages_to_scan=100000, sleep_millisecs=0, 3 runs each; churners: 4
threads mmap/munmaping 256 MiB in the same mm. Microbenchmark as
posted in [3]; relative numbers only, TCG is not native hardware):

                       0 churners      4 churners (mean)   
  7.3-rc2              1.21-1.41 s     90.9 s             
  + patch 2/3          1.41 s          48.7 s   (-46%)   
  + patch 2/3 + 3/3    1.21-1.41 s     30.1 s   (-67%)  


 1. mm/pagewalk: allow folio_walk_start() under a vma read lock

 2. mm/ksm: use the VMA lock when looking up mergeable pages

 3. mm/ksm: scan VMAs with per-VMA locks

Link:https://lore.kernel.org/all/5cd47bc3-bc3d-473c-80d0-8be8b7b79881@xxxxxxx/

Longlong Xia (3):
  mm/pagewalk: allow folio_walk_start() under a vma read lock
  mm/ksm: use the VMA lock when looking up mergeable pages
  mm/ksm: scan VMAs with per-VMA locks

 include/linux/pagewalk.h |   3 +
 mm/ksm.c                 | 166 ++++++++++++++++++++++++++++++++++++---
 mm/pagewalk.c            |  11 ++-
 3 files changed, 166 insertions(+), 14 deletions(-)

base-commit: df2908090cda368b01ff43709f51890076c56157
-- 
2.43.0