[RFC PATCH v2 0/4] mm/swap: reserve swap areas for deliberate offload

[Matthias Goergens]

This RFC adds offload-only swap areas for deliberate cold-page offload to
backends unsuitable for pressure reclaim. ZFS zvol swap has documented
deadlocks under memory pressure [3]; compressed swap is another target
because writes may need memory despite free logical slots.

Explicit proactive reclaim can use these areas alongside conventional
swap, ordered by priority. Ordinary reclaim cannot initiate non-zero
writes to them, including through retained swap entries. Conventional
capacity is not reserved for emergencies. Operators choose the policy;
the kernel does not measure headroom or make an allocating backend safe.

TMO/Senpai [4] and DAMON_RECLAIM [5] are related cold-page reclaim
approaches. This RFC admits offload-only swap through memory.reclaim and
per-node reclaim, but not DAMON. Other related work includes per-cgroup
zswap writeback control [6], Virtual Swap Space v4 [1] and swap tiers v10
[2].

The util-linux companion [8] proposes swapon --offload-only and an fstab
option. Older swapon silently ignores the fstab token, so persistent
activation needs discussion. Apply the separate i915 fix [9] first to avoid
a pre-existing folio-lock leak when shmem writeback is skipped.

The series fixes zram selftest device tracking and error reporting, adds
the swap policy with DRM eligibility checks, and tests routing, workingset
activation and retained-entry write refusal. It is based on mm-new at
995829088503.

Feedback is particularly welcome on:

- Activation-time eligibility versus backend placement or migration:
  is retained-entry refusal, with possible reclaim churn or OOM,
  acceptable without migration?
- Eligible-capacity accounting, including overcommit limits and OOM
  scoring; cache recovery, cluster invalidation and workingset activation.
- Persistent activation and visibility: current swap listings do not
  expose the policy.

Validation includes x86 builds, affected arm64 MTE objects and builds
without swap or memory cgroups. QEMU tests covered routing, retained-entry
recovery, data integrity and cleanup, with negative controls for write
refusal and teardown failure.

I have also been using this policy on my own machine with experimental
bcachefs swap support, with no problems observed so far. Deliberate stress
testing is confined to VMs; I do not deliberately stress-test this machine.

Changes since v1, incorporating Sashiko's public review [7]:

- Improve selftest isolation, retained-swap measurement and memlock skips.
- Track allocated zram devices, wait for udev probes and report cleanup
  errors without deleting data through a mount that could not be released.
- Clarify activation, capacity reporting and conventional fallback limits.

[1] https://patchew.org/linux/20260825153238.2695446-1-nphamcs@xxxxxxxxx/
[2] https://patchew.org/linux/20260713025644.170839-1-youngjun.park@xxxxxxx/
[3] https://openzfs.github.io/openzfs-docs/Project%20and%20Community/FAQ.html#using-a-zvol-for-a-swap-device-on-linux
[4] https://engineering.fb.com/2022/06/20/data-infrastructure/transparent-memory-offloading-more-memory-at-a-fraction-of-the-cost-and-power/
[5] https://www.kernel.org/doc/html/latest/admin-guide/mm/damon/reclaim.html
[6] https://www.kernel.org/doc/html/latest/admin-guide/mm/zswap.html
[7] https://sashiko.dev/#/patchset/20260914104501.3960616-1-matthias.goergens@xxxxxxxxx
[8] https://github.com/util-linux/util-linux/pull/4633
[9] https://lore.kernel.org/all/20260914105606.3997649-1-matthias.goergens@xxxxxxxxx/

v1: https://lore.kernel.org/all/20260914104501.3960616-1-matthias.goergens@xxxxxxxxx/

Matthias Goergens (4):
  selftests: zram: track owned devices and report cleanup failures
  mm: restrict offload-only swap to proactive reclaim
  selftests: zram: cover offload-only swap policy
  selftests: zram: cover retained offload-only entries

 Documentation/mm/swap.rst                     | 105 ++++
 drivers/gpu/drm/i915/gem/i915_gem_shrinker.c  |   2 +-
 .../gpu/drm/i915/gem/selftests/huge_pages.c   |   6 +-
 drivers/gpu/drm/msm/msm_gem_shrinker.c        |   2 +-
 drivers/gpu/drm/panthor/panthor_gem.c         |   2 +-
 drivers/gpu/drm/ttm/ttm_backup.c              |   2 +-
 drivers/gpu/drm/xe/tests/xe_bo.c              |   2 +-
 include/linux/swap.h                          |  29 +-
 include/linux/vm_event_item.h                 |   1 +
 mm/memcontrol.c                               |  20 +-
 mm/page_io.c                                  |  24 +-
 mm/swapfile.c                                 | 123 ++++-
 mm/vmscan.c                                   |  31 +-
 mm/vmstat.c                                   |   1 +
 tools/testing/selftests/zram/.gitignore       |   2 +
 tools/testing/selftests/zram/Makefile         |   4 +-
 tools/testing/selftests/zram/README           |  20 +-
 tools/testing/selftests/zram/config           |  10 +-
 tools/testing/selftests/zram/settings         |   1 +
 tools/testing/selftests/zram/swap_offload.c   | 425 +++++++++++++++++
 .../selftests/zram/workingset_offload.c       | 207 ++++++++
 tools/testing/selftests/zram/zram.sh          |   9 +
 tools/testing/selftests/zram/zram01.sh        |   9 +-
 tools/testing/selftests/zram/zram02.sh        |   9 +-
 tools/testing/selftests/zram/zram03.sh        | 174 +++++++
 tools/testing/selftests/zram/zram04.sh        | 164 +++++++
 tools/testing/selftests/zram/zram05.sh        | 447 ++++++++++++++++++
 tools/testing/selftests/zram/zram_lib.sh      | 189 ++++++--
 28 files changed, 1926 insertions(+), 94 deletions(-)
 create mode 100644 tools/testing/selftests/zram/settings
 create mode 100644 tools/testing/selftests/zram/swap_offload.c
 create mode 100644 tools/testing/selftests/zram/workingset_offload.c
 create mode 100755 tools/testing/selftests/zram/zram03.sh
 create mode 100755 tools/testing/selftests/zram/zram04.sh
 create mode 100755 tools/testing/selftests/zram/zram05.sh

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2.55.0