[PATCH v1 0/7] mm/ksm: break_ksm() cleanups and fixes

[David Hildenbrand]

This series cleans up and fixes break_ksm(). In summary, we no longer
use fake write faults to break COW but instead FAULT_FLAG_UNSHARE. Further,
we move away from using follow_page() [that we can hopefully remove
completely at one point] and use new walk_page_range_vma() instead.

Fortunately, we can get rid of VM_FAULT_WRITE and FOLL_MIGRATION in common
code now.

Add a selftest to measure MADV_UNMERGEABLE performance. In my setup
(AMD Ryzen 9 3900X), running the KSM selftest to test unmerge performance
on 2 GiB (taskset 0x8 ./ksm_tests -D -s 2048), this results in a
performance degradation of ~8% -- 9% (old: ~5250 MiB/s, new: ~4800 MiB/s).
I don't think we particularly care for now, but it's good to be aware
of the implication.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>

David Hildenbrand (7):
  selftests/vm: add test to measure MADV_UNMERGEABLE performance
  mm/ksm: simplify break_ksm() to not rely on VM_FAULT_WRITE
  mm: remove VM_FAULT_WRITE
  mm/ksm: fix KSM COW breaking with userfaultfd-wp via
    FAULT_FLAG_UNSHARE
  mm/pagewalk: add walk_page_range_vma()
  mm/ksm: convert break_ksm() to use walk_page_range_vma()
  mm/gup: remove FOLL_MIGRATION

 include/linux/mm.h                     |   1 -
 include/linux/mm_types.h               |   3 -
 include/linux/pagewalk.h               |   3 +
 mm/gup.c                               |  55 ++-----------
 mm/huge_memory.c                       |   2 +-
 mm/ksm.c                               | 103 +++++++++++++++++++------
 mm/memory.c                            |   9 +--
 mm/pagewalk.c                          |  27 +++++++
 tools/testing/selftests/vm/ksm_tests.c |  76 +++++++++++++++++-
 9 files changed, 192 insertions(+), 87 deletions(-)

-- 
2.37.3