[PATCH v4 0/6] Swap-out mTHP without splitting

[Ryan Roberts]

Hi All,

This series adds support for swapping out multi-size THP (mTHP) without needing
to first split the large folio via split_huge_page_to_list_to_order(). It
closely follows the approach already used to swap-out PMD-sized THP.

There are a couple of reasons for swapping out mTHP without splitting:

  - Performance: It is expensive to split a large folio and under extreme memory
    pressure some workloads regressed performance when using 64K mTHP vs 4K
    small folios because of this extra cost in the swap-out path. This series
    not only eliminates the regression but makes it faster to swap out 64K mTHP
    vs 4K small folios.

  - Memory fragmentation avoidance: If we can avoid splitting a large folio
    memory is less likely to become fragmented, making it easier to re-allocate
    a large folio in future.

  - Performance: Enables a separate series [4] to swap-in whole mTHPs, which
    means we won't lose the TLB-efficiency benefits of mTHP once the memory has
    been through a swap cycle.

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 sufficient.


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

I've run some swap performance tests on Ampere Altra VM (arm64) with 8 CPUs. The
VM is set up with a 35G block ram device as the swap device and the test is run
from inside a memcg limited to 40G memory. I've then run `usemem` from
vm-scalability with 70 processes, each allocating and writing 1G of memory. I've
repeated everything 6 times and taken the mean performance improvement relative
to 4K page baseline:

| alloc size |            baseline |       + this series |
|            |  v6.6-rc4+anonfolio |                     |
|:-----------|--------------------:|--------------------:|
| 4K Page    |                0.0% |                1.4% |
| 64K THP    |              -14.6% |               44.2% |
| 2M THP     |               87.4% |               97.7% |

So with this change, the 64K swap performance goes from a 15% regression to a
44% improvement. 4K and 2M swap improves slightly too.

This test also acts as a good stress test for swap and, more generally mm. A
couple of existing bugs were found as a result [5] [6].


---
The series applies against mm-unstable (d7182786dd0a). Although I've
additionally been running with a couple of extra fixes to avoid the issues at
[6].


Changes since v3 [3]
====================

 - Renamed SWAP_NEXT_NULL -> SWAP_NEXT_INVALID (per Huang, Ying)
 - Simplified max offset calculation (per Huang, Ying)
 - Reinstated struct percpu_cluster to contain per-cluster, per-order `next`
   offset (per Huang, Ying)
 - Removed swap_alloc_large() and merged its functionality into
   scan_swap_map_slots() (per Huang, Ying)
 - Avoid extra cost of folio ref and lock due to removal of CLUSTER_FLAG_HUGE
   by freeing swap entries in batches (see patch 2) (per DavidH)
 - vmscan splits folio if its partially mapped (per Barry Song, DavidH)
 - Avoid splitting in MADV_PAGEOUT path (per Barry Song)
 - Dropped "mm: swap: Simplify ssd behavior when scanner steals entry" patch
   since it's not actually a problem for THP as I first thought.


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).


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).


[1] https://lore.kernel.org/linux-mm/20231010142111.3997780-1-ryan.roberts@xxxxxxx/
[2] https://lore.kernel.org/linux-mm/20231017161302.2518826-1-ryan.roberts@xxxxxxx/
[3] https://lore.kernel.org/linux-mm/20231025144546.577640-1-ryan.roberts@xxxxxxx/
[4] https://lore.kernel.org/linux-mm/20240304081348.197341-1-21cnbao@xxxxxxxxx/
[5] https://lore.kernel.org/linux-mm/20240311084426.447164-1-ying.huang@xxxxxxxxx/
[6] https://lore.kernel.org/linux-mm/79dad067-1d26-4867-8eb1-941277b9a77b@xxxxxxx/

Thanks,
Ryan


Ryan Roberts (6):
  mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags
  mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()
  mm: swap: Simplify struct percpu_cluster
  mm: swap: Allow storage of all mTHP orders
  mm: vmscan: Avoid split during shrink_folio_list()
  mm: madvise: Avoid split during MADV_PAGEOUT and MADV_COLD

 include/linux/pgtable.h |  28 ++++
 include/linux/swap.h    |  33 +++--
 mm/huge_memory.c        |   3 -
 mm/internal.h           |  48 +++++++
 mm/madvise.c            | 101 ++++++++------
 mm/memory.c             |  13 +-
 mm/swapfile.c           | 298 ++++++++++++++++++++++------------------
 mm/vmscan.c             |   9 +-
 8 files changed, 332 insertions(+), 201 deletions(-)

--
2.25.1