[PATCH v6 0/3] Reclaim lazyfree THP without splitting

[Lance Yang]

Hi all,

This series adds support for reclaiming PMD-mapped THP marked as lazyfree
without needing to first split the large folio via split_huge_pmd_address().

When the user no longer requires the pages, they would use madvise(MADV_FREE)
to mark the pages as lazy free. Subsequently, they typically would not re-write
to that memory again.

During memory reclaim, if we detect that the large folio and its PMD are both
still marked as clean and there are no unexpected references(such as GUP), so we
can just discard the memory lazily, improving the efficiency of memory
reclamation in this case.

Performance Testing
===================

On an Intel i5 CPU, reclaiming 1GiB of lazyfree THPs using
mem_cgroup_force_empty() results in the following runtimes in seconds
(shorter is better):

--------------------------------------------
|     Old       |      New       |  Change  |
--------------------------------------------
|   0.683426    |    0.049197    |  -92.80% |
--------------------------------------------

---

Changes since v5 [5]
====================
 - mm/rmap: remove duplicated exit code in pagewalk loop
    - Pick RB from Baolin Wang - thanks!
 - mm/mlock: check for THP missing the mlock in try_to_unmap_one()
    - Merge this patch into patch 2 (per Baolin Wang)
 - mm/vmscan: avoid split lazyfree THP during shrink_folio_list()
    - Mark a folio as being backed by swap space if the folio or its PMD
      was redirtied (per Baolin Wang)
    - Use pmdp_huge_clear_flush() to get and flush a PMD entry
      (per Baolin Wang)

Changes since v4 [4]
====================
 - mm/rmap: remove duplicated exit code in pagewalk loop
    - Pick RB from Zi Yan - thanks!
 - mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop
    - Remove the redundant alignment (per Baolin Wang)
    - Set pvmw.ptl to NULL after unlocking the PTL (per Baolin Wang)
 - mm/mlock: check for THP missing the mlock in try_to_unmap_one()
    - Check whether the mlock of PMD-mapped THP was missed
      (suggested by Baolin Wang)
 - mm/vmscan: avoid split lazyfree THP during shrink_folio_list()
    - No need to check the TTU_SPLIT_HUGE_PMD flag for unmap_huge_pmd_locked()
      (per Zi Yan)
    - Drain the local mlock batch after folio_remove_rmap_pmd()
      (per Baolin Wang)

Changes since v3 [3]
====================
 - mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop
    - Resolve compilation errors by handling the case where
      CONFIG_PGTABLE_HAS_HUGE_LEAVES is undefined (thanks to SeongJae Park)
 - mm/vmscan: avoid split lazyfree THP during shrink_folio_list()
    - Remove the unnecessary conditional compilation directives
      (thanks to Barry Song)
    - Resolve compilation errors due to undefined references to
      unmap_huge_pmd_locked and split_huge_pmd_locked (thanks to Barry)

Changes since v2 [2]
====================
 - Update the changelog (thanks to David Hildenbrand)
 - Support try_to_unmap_one() to unmap PMD-mapped folios
   (thanks a lot to David Hildenbrand and Zi Yan)

Changes since v1 [1]
====================
 - Update the changelog
 - Follow the exact same logic as in try_to_unmap_one() (per David Hildenbrand)
 - Remove the extra code from rmap.c (per Matthew Wilcox)

[1] https://lore.kernel.org/linux-mm/20240417141111.77855-1-ioworker0@xxxxxxxxx
[2] https://lore.kernel.org/linux-mm/20240422055213.60231-1-ioworker0@xxxxxxxxx
[3] https://lore.kernel.org/linux-mm/20240429132308.38794-1-ioworker0@xxxxxxxxx
[4] https://lore.kernel.org/linux-mm/20240501042700.83974-1-ioworker0@xxxxxxxxx
[5] https://lore.kernel.org/linux-mm/20240513074712.7608-1-ioworker0@xxxxxxxxx

Lance Yang (3):
  mm/rmap: remove duplicated exit code in pagewalk loop
  mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop
  mm/vmscan: avoid split lazyfree THP during shrink_folio_list()

 include/linux/huge_mm.h |  15 +++++
 mm/huge_memory.c        | 122 +++++++++++++++++++++++++++++++++-------
 mm/rmap.c               |  83 ++++++++++++++++-----------
 3 files changed, 167 insertions(+), 53 deletions(-)

-- 
2.33.1