[PATCH v2 0/4] KSM: introduce only_merge_zero_pages

["Xu Xin (vivo)" <xu.xin@linux.dev>]

Hi all,

This series adds a new KSM sysfs attribute, only_merge_zero_pages, which
makes ksmd merge only empty (zeroed) anonymous pages with the kernel
zero page and skip the stable/unstable trees for pages that  carry data.

Patch 1 adds the attribute and the early return in cmp_and_merge_page().

Patch 2 (co-developed with Jinjiang Tu) goes one step further and skips
non-zero pages already in the scan path, before an rmap_item is
allocated for them, which is what makes the mode practical on
large-memory workloads with mostly non-mergeable anonymous memory.

Patch 3 consolidates the ksm_stat readers in vm_util.c into a single
ksm_get_self_stat() helper (pure refactor). Patch 4 adds a functional
selftest.

Changelog
=========
v1 -> v2

- Patch 1: Take ksm_thread_mutex around the check-and-set in both
           use_zero_pages_store() and only_merge_zero_pages_store(), so
           that a concurrent write to the two attributes cannot leave
           use_zero_pages=0 and only_merge_zero_pages=1, a state in which
           ksmd would scan and checksum pages but never merge any.

- New patch 2, 3 and 4 are added.

Motivation
==========

On an Android device, I measured the KSM savings potential on 15.7 GiB
RAM, ~1100 processes, a typical usage session with several video apps
running) using a userspace survey tool that walks /proc/<pid>/maps +
/proc/<pid>/pagemap, deduplicates anonymous pages by their physical
frame number (PFN), and hashes page contents to find identical pages.
Key results (two samples taken 10 minutes apart):

  Category              Resident    Savings (upper bound)
  -------------------   ---------   ---------------------
  anon, non-zero pages  2.9G        67-74 MiB (2.3-2.4%)
  anon, zero pages      -           281-424 MiB (9.5-13.9%)

In other scenario with a MySQL instance from huawei, there are around
15 GB consumed by hashtables, which contain many zero-filled pages that
are eligible for merging to reduce memory footprint [1].

[1] https://lore.kernel.org/all/2a642af6-b97a-4f24-9889-1844812de4cc@huawei.com/


Xu Xin (vivo) (4):
  ksm: add only_merge_zero_pages to merge only empty anonymous pages
  ksm: skip rmap_item allocation for non-zero pages under
    only_merge_zero_pages
  selftests/ksm: consolidate ksm_stat readers into ksm_get_self_stat()
  selftests/ksm: add test for only_merge_zero_pages

 Documentation/admin-guide/mm/ksm.rst          |  40 ++++-
 mm/ksm.c                                      | 149 ++++++++++++++++++
 .../selftests/mm/ksm_functional_tests.c       | 135 ++++++++++++++--
 tools/testing/selftests/mm/vm_util.c          |  40 ++---
 tools/testing/selftests/mm/vm_util.h          |   3 +-
 5 files changed, 327 insertions(+), 40 deletions(-)

-- 
2.34.1