support batch checking of references and unmapping for large folios

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(-)

2.47.3