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 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          | 31 ++++++++++++++++
 mm/rmap.c                        | 38 ++++++++++++++++----
 5 files changed, 125 insertions(+), 38 deletions(-)

-- 
2.47.3

Andrew -

I know this has had a lot of attention, but can we hold off on sending this
upstream until either David or I have had a chance to review it?

Also note that Dev has discovered an issue with how this interacts with the
accursed uffd-wp logic (see [0]) so series needs a respin anyway.

Thanks, Lorenzo

[0]: https://lore.kernel.org/linux-mm/20260116082721.275178-1-dev.jain@arm.com/


On Fri, Dec 26, 2025 at 02:07:54PM +0800, Baolin Wang wrote:
> 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 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          | 31 ++++++++++++++++
>  mm/rmap.c                        | 38 ++++++++++++++++----
>  5 files changed, 125 insertions(+), 38 deletions(-)
>
> --
> 2.47.3
>

On 1/16/26 09:41, Lorenzo Stoakes wrote:
> Andrew -
> 
> I know this has had a lot of attention, but can we hold off on sending this
> upstream until either David or I have had a chance to review it?

Ah, I didn't read your mail before I sent mine. +1 ;)

-- 
Cheers

David

On 12/26/25 07:07, Baolin Wang wrote:
> 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.


Baolin, I'm intending to review this, but it might still take me a bit 
until I get to it. (PTO/vacation and other fun)

-- 
Cheers

David