[RFC PATCH 0/3] mm/swap: reserve swap areas for deliberate offload

[Matthias Goergens]

Some swap backends are useful for deliberate cold-page offload but are poor
dependencies for ordinary reclaim under acute memory pressure. Swap on ZFS
zvols is an existing example, with documented deadlocks under memory
pressure [3]. Compressed swap is another attractive target: accepting a
write may require memory for compression or storage even when logical swap
slots remain free.

This series adds an offload-only swap-area policy for such deployments,
alongside a conventional swap reserve for pressure reclaim.

Proactive offloading is already established by TMO/Senpai [4], which uses
pressure feedback to offload cold pages to compressed memory or SSDs.
DAMON_RECLAIM [5] also uses free-memory watermarks to suspend proactive
reclaim under severe pressure. These control when reclaim runs; this RFC
restricts which swap areas ordinary reclaim may write, including through
retained swap entries. The existing memory.zswap.writeback control [6]
restricts backing-swap writes per cgroup, rather than reserving individual
swap areas for explicitly admitted offload.

The minimum contract is that ordinary reclaim must not initiate a new,
non-zero backend write to an offload-only area. Reads, swapoff, zero-page
metadata, architecture preparation, and I/O that was already admitted remain
valid. If no eligible swap remains, existing reclaim and OOM behaviour applies.
This RFC does not promise a performance result or define a new workload policy.

Cgroup v2 memory.reclaim and per-node reclaim establish admission. Manual
MGLRU eviction currently does too, but its debugfs interface is not stable
ABI. Ordinary direct reclaim, kswapd, MADV_PAGEOUT and DAMON reclaim do not
gain access. This is an explicit operator policy; the kernel does not measure
physical headroom or automatically classify backends.

The three patches are based on mm-new as refreshed on 11 September, at
995829088503:

1. The complete swap-area policy, including eligible-capacity accounting and
   recovery of eligible cache entries.
2. zram03/zram04 coverage for reclaim routing, zswap bypass, discard policy
   and file workingset activation.
3. zram05 coverage for retained swap entries, including ordinary-write
   refusal, proactive recovery, data integrity and checked teardown.

The implementation is intended to make these questions reviewable:

1. Should pressure eligibility be a durable property activated with the
   physical swap area, or should it be selected late during backend migration?
2. Do provenance, allocation filtering, eligible capacity, and retained-entry
   refusal provide a useful minimum contract for userspace and controllers?
3. Is redirtying or activating a retained dirty folio, with possible churn or
   OOM, acceptable, or is backend migration a prerequisite?
4. How should persistent activation fail closed and remain auditable when
   /proc/swaps and swapon --show do not expose the policy?
5. Should an ineligible fast-path cluster be invalidated immediately, and
   should an offload-only area recover its own cache entries based on eligible
   fullness rather than global swap fullness?
6. Should the workingset activation heuristic continue to count offload-only
   capacity when pressure reclaim cannot allocate from it?

Build checks passed on x86 with i915 built-in or modular, and with swap or
memory cgroups disabled. The affected arm64 MTE objects also compiled.

The routing and workingset tests passed in x86 QEMU guests. In the
retained-entry test, ordinary reclaim wrote nothing to offload-only swap;
proactive reclaim then wrote the dirty folio, and all data verified
correctly. With the write guard removed, the same test failed because
ordinary reclaim wrote to the offload-only area.

Persistent fstab encoding remains unsettled. When hibernation is about to
save an image, an active offload-only area named by resume= is rejected as
the image target with -ENODEV. Virtual Swap Space v4 [1] separates swap
entries from their physical backing, while swap tiers v10 [2] proposes
per-cgroup backend selection. These raise the question of whether offload
eligibility belongs to physical swap areas or a later placement decision;
this RFC does not assume those interfaces are settled.

An explicit-only util-linux companion implementing swapon --offload-only
is prepared for separate submission.

Feedback on the eligibility contract, its placement in the swap stack,
and the unresolved ABI and reclaim-policy questions would be welcome.

[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

Matthias Goergens (3):
  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                     |  87 ++++
 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                                 | 126 +++++-
 mm/vmscan.c                                   |  31 +-
 mm/vmstat.c                                   |   1 +
 tools/testing/selftests/zram/.gitignore       |   2 +
 tools/testing/selftests/zram/Makefile         |   4 +-
 tools/testing/selftests/zram/config           |  10 +-
 tools/testing/selftests/zram/swap_offload.c   | 402 ++++++++++++++++++
 .../selftests/zram/workingset_offload.c       | 206 +++++++++
 tools/testing/selftests/zram/zram03.sh        | 166 ++++++++
 tools/testing/selftests/zram/zram04.sh        | 149 +++++++
 tools/testing/selftests/zram/zram05.sh        | 360 ++++++++++++++++
 tools/testing/selftests/zram/zram_lib.sh      |  28 ++
 23 files changed, 1620 insertions(+), 42 deletions(-)
 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


base-commit: 9958290885035431e2494159f555e332c3c14906
-- 
2.55.0