[PATCH v6 0/5] support batch checking of references and unmapping for large folios

[Baolin Wang <baolin.wang@linux.alibaba.com>]

Currently, folio_referenced_one() always checks the young flag for each PTE
sequentially, which is inefficient for large folios. This inefficiency is
especially noticeable when reclaiming clean file-backed large folios, where
folio_referenced() is observed as a significant performance hotspot.

Moreover, on Arm architecture, which supports contiguous PTEs, there is already
an optimization to clear the young flags for PTEs within a contiguous range.
However, this is not sufficient. We can extend this to perform batched operations
for the entire large folio (which might exceed the contiguous range: CONT_PTE_SIZE).

Similar to folio_referenced_one(), we can also apply batched unmapping for large
file folios to optimize the performance of file folio reclamation. By supporting
batched checking of the young flags, flushing TLB entries, and unmapping, I can
observed a significant performance improvements in my performance tests for file
folios reclamation. Please check the performance data in the commit message of
each patch.

Run stress-ng and mm selftests, no issues were found.

Patch 1: Add a new generic batched PTE helper that supports batched checks of
the references for large folios.
Patch 2 - 3: Preparation patches.
patch 4: Implement the Arm64 arch-specific clear_flush_young_ptes().
Patch 5: Support batched unmapping for file large folios.

Changes from v5:
 - Collect reviewed tags from Ryan, Harry and David. Thanks.
 - Fix some coding style issues (per David).
 - Skip batched unmapping for uffd case, reported by Dev. Thanks.

Changes from v4:
 - Fix passing the incorrect 'CONT_PTES' for non-batched APIs.
 - Rename ptep_clear_flush_young_notify() to clear_flush_young_ptes_notify() (per Ryan).
 - Fix some coding style issues (per Ryan).
 - Add reviewed tag from Ryan. Thanks.

Changes from v3:
 - Fix using an incorrect parameter in ptep_clear_flush_young_notify()
   (per Liam).

Changes from v2:
 - Rearrange the patch set (per Ryan).
 - Add pte_cont() check in clear_flush_young_ptes() (per Ryan).
 - Add a helper to do contpte block alignment (per Ryan).
 - Fix some coding style issues (per Lorenzo and Ryan).
 - Add more comments and update the commit message (per Lorenzo and Ryan).
 - Add acked tag from Barry. Thanks. 

Changes from v1:
 - Add a new patch to support batched unmapping for file large folios.
 - Update the cover letter


Baolin Wang (5):
  mm: rmap: support batched checks of the references for large folios
  arm64: mm: factor out the address and ptep alignment into a new helper
  arm64: mm: support batch clearing of the young flag for large folios
  arm64: mm: implement the architecture-specific
    clear_flush_young_ptes()
  mm: rmap: support batched unmapping for file large folios

 arch/arm64/include/asm/pgtable.h | 23 ++++++++----
 arch/arm64/mm/contpte.c          | 62 ++++++++++++++++++++------------
 include/linux/mmu_notifier.h     |  9 ++---
 include/linux/pgtable.h          | 35 ++++++++++++++++++
 mm/rmap.c                        | 38 ++++++++++++++++----
 5 files changed, 129 insertions(+), 38 deletions(-)

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2.47.3