[PATCH v4 0/5] mm/ksm: keep smart scan from losing duplicate pages

[Qiliang Yuan]

With smart scan, which is on by default, two identical pages that become
mergeable some scans apart can stay unmerged for as long as they live:
each page times its skipped scans on its own, and once both are old they
may never be looked at in the same scan, which merging through the
unstable tree requires. This was first noticed as intermittent failures
of ksm_tests "KSM NUMA merging" in run_vmtests.sh.

Patch 1 picks the skipped scans by the global scan number, so all old
pages are looked at together every eighth scan. Over 420 pairs made
mergeable 0 to 20 scans apart, 361 and 334 merged before on two boots
and all of them after, at the cost of about 10% more ksmd time on memory
that never merges. Pairs whose pages are both old before they become
identical fare worse: of 1800 such pairs, 200 merged before, still 200
after 256 scans, and all 1800 within 32 scans after.

Patch 2 documents how smart scan picks the scans to skip.

Patch 3 makes the KSM helpers in vm_util fail on short reads and
writes, which could make ksm_functional_tests hang or fail cases that
should be skipped. Patch 4 moves waiting for KSM full scans into a
vm_util helper, and patch 5 adds a ksm_functional_tests case that fails
20/20 without patch 1 and 0/20 with it, on both boots.

Signed-off-by: Qiliang Yuan <odys.yuan@xxxxxxxxx>
---
V3 -> V4:
- Make the KSM helpers in vm_util fail on short reads and writes, in a
  new patch 3, and the smart_scan helpers of patch 5 likewise
- Add Cc: stable to patch 1
- Add Reviewed-by and Tested-by tags

V2 -> V3:
- Document smart scan in Documentation/mm/ksm.rst, as Xu Xin suggested
- Wait for full scans and switch smart scan through vm_util helpers, as
  David suggested
- Rebase onto v7.4-rc1-345-geedaa7d61f1e and retest there
- Measure ksmd CPU time with schedstat, which puts the cost at about 10%

V1 -> V2:
- Rebase onto v7.4-rc1-132-g05441d697f7f and retest there
- Drop the ksm_tests -N -m 1 numbers, as it no longer fails on this base
  without the fix
- Update the ksmd cost to 10-12%, measured on this base

v3: https://lore.kernel.org/r/20261107-bug-mm-ksm-smart-scan-align-skips-v3-0-16d5ce8b5fd6@xxxxxxxxx
v2: https://lore.kernel.org/r/20261105-bug-mm-ksm-smart-scan-align-skips-v2-0-8a9780b6844e@xxxxxxxxx
v1: https://lore.kernel.org/r/20261016-bug-mm-ksm-smart-scan-align-skips-v1-0-2f91d76416be@xxxxxxxxx

---
Qiliang Yuan (5):
      mm/ksm: align smart scan skips with the scan number
      docs/mm: ksm: describe how smart scan picks the scans to skip
      selftests/mm: fail KSM helpers on short reads and writes
      selftests/mm: move waiting for KSM full scans into a helper
      selftests/mm: cover merging KSM pages aged by smart scan

 Documentation/mm/ksm.rst                          | 16 ++++
 mm/ksm.c                                          | 24 +++---
 tools/testing/selftests/mm/ksm_functional_tests.c | 89 ++++++++++++++++++++++-
 tools/testing/selftests/mm/vm_util.c              | 89 ++++++++++++++++++-----
 tools/testing/selftests/mm/vm_util.h              |  3 +
 5 files changed, 188 insertions(+), 33 deletions(-)
---
base-commit: eedaa7d61f1ec8ae053a9ece0d553785350ac486
change-id: 20261016-bug-mm-ksm-smart-scan-align-skips-cd01e5a9889a

Best regards,
-- 
Qiliang Yuan <odys.yuan@xxxxxxxxx>