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

Ryan Roberts
2024-03-11T15:01:10Z

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

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