[PATCH v3 0/4] Swap-out small-sized THP without splitting

[Ryan Roberts <ryan.roberts@arm.com>]

Hi All,

This is v3 of a series to add support for swapping out small-sized THP without
needing to first split the large folio via __split_huge_page(). It closely
follows the approach already used by PMD-sized THP.

"Small-sized THP" is an upcoming feature that enables performance improvements
by allocating large folios for anonymous memory, where the large folio size is
smaller than the traditional PMD-size. See [3].

In some circumstances I've observed a performance regression (see patch 2 for
details), and this series is an attempt to fix the regression in advance of
merging small-sized THP support.

I've done what I thought was the smallest change possible, and as a result, this
approach is only employed when the swap is backed by a non-rotating block device
(just as PMD-sized THP is supported today). Discussion against the RFC concluded
that this is probably sufficient.

The series applies against mm-unstable (1a3c85fa684a)


Changes since v2 [2]
====================

 - Reuse scan_swap_map_try_ssd_cluster() between order-0 and order > 0
   allocation. This required some refactoring to make everything work nicely
   (new patches 2 and 3).
 - Fix bug where nr_swap_pages would say there are pages available but the
   scanner would not be able to allocate them because they were reserved for the
   per-cpu allocator. We now allow stealing of order-0 entries from the high
   order per-cpu clusters (in addition to exisiting stealing from order-0
   per-cpu clusters).

Thanks to Huang, Ying for the review feedback and suggestions!


Changes since v1 [1]
====================

 - patch 1:
    - Use cluster_set_count() instead of cluster_set_count_flag() in
      swap_alloc_cluster() since we no longer have any flag to set. I was unable
      to kill cluster_set_count_flag() as proposed against v1 as other call
      sites depend explicitly setting flags to 0.
 - patch 2:
    - Moved large_next[] array into percpu_cluster to make it per-cpu
      (recommended by Huang, Ying).
    - large_next[] array is dynamically allocated because PMD_ORDER is not
      compile-time constant for powerpc (fixes build error).


Thanks,
Ryan

P.S. I know we agreed this is not a prerequisite for merging small-sized THP,
but given Huang Ying had provided some review feedback, I wanted to progress it.
All the actual prerequisites are either complete or being worked on by others.


[1] https://lore.kernel.org/linux-mm/20231010142111.3997780-1-ryan.roberts@arm.com/
[2] https://lore.kernel.org/linux-mm/20231017161302.2518826-1-ryan.roberts@arm.com/
[3] https://lore.kernel.org/linux-mm/15a52c3d-9584-449b-8228-1335e0753b04@arm.com/


Ryan Roberts (4):
  mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags
  mm: swap: Remove struct percpu_cluster
  mm: swap: Simplify ssd behavior when scanner steals entry
  mm: swap: Swap-out small-sized THP without splitting

 include/linux/swap.h |  31 +++---
 mm/huge_memory.c     |   3 -
 mm/swapfile.c        | 232 ++++++++++++++++++++++++-------------------
 mm/vmscan.c          |  10 +-
 4 files changed, 149 insertions(+), 127 deletions(-)

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2.25.1