[PATCH v3 0/3] dm zoned: fix two idle reclaim problems

[Kyrylo Biryuk]

Changes in v3:
- Patch 2: reworded the comment and dropped the redundant
  dmz_nr_cache_zones() test, so the condition fits on one line
  (Damien Le Moal). Patches 1 and 3 are unchanged.
- From and Signed-off-by now carry my full name instead of the GitHub
  handle, as you preferred on v2. Thanks for checking.

Changes in v2:
- No code changes; only my author address changed.

These patches fix two problems in dm-zoned's reclaim worker, found while
running md RAID5 on three dm-zoned targets on 27 TB host-managed SMR drives
(WD Ultrastar DC HC680), each target with a regular cache device.

1. On a target with cache zones and no free sequential zone, an idle
   target keeps copying random zones into other random zones, forever
   (patch 2; patch 1 makes the resulting -ENOSPC back off instead of
   spinning). After a full md rebuild every chunk is mapped, so this
   happens right after a rebuild: the idle member read and wrote about
   72 MB/s nonstop for hours.

2. After a reclaim pass that ends while the target is busy, the idle poll
   is never re-armed, so zones filled under load are not reclaimed when the
   target goes idle (patch 3). Users notice it as "the buffer never drains";
   the workaround has been a timer sending "dmsetup message <dev> 0 reclaim".

Testing: the three patches were built as an out-of-tree dm-zoned module for
7.2.6 (the changed code is identical in current master) and tested
- on emulated host-managed drives (tcmu-runner ZBC handler) in a VM: the
  idle copy loop reproduced with the stock module (340 MB/s on an idle
  target, zone counters unchanged), 0 MB/s with the patches;
- on the three real drives since 2026-10-05: crash test, three 1.5 TiB
  write benchmarks, and eight conversions between single-device and
  cache-device layouts; emptied zones were reclaimed within minutes of the
  targets going idle without the reclaim timer.
- with a reproducer that needs no special hardware (scsi_debug zbc=managed,
  64 MiB zones, plus a 1 GiB loop cache device): stock module ~600 MB/s of
  read+write on the idle target with unchanged zone counters, patched
  module 0 MB/s:
  https://github.com/illofspeed/synology-host-managed-smr/tree/main/repro
Not done: a test on current master itself (only 7.2.6), and a rebuild onto a
cached member with the patches (the first problem was seen after a rebuild
with the stock module).

Related, not addressed here: the double-free in dmz_load_sb() reported on
dm-devel on 2026-05-30.

The problems were found and the patches written with the help of an AI
coding assistant (hence Assisted-by); the analysis, the tests on real
hardware and this description were reviewed by me.

Kyrylo Biryuk (3):
  dm zoned: back off when reclaim finds no destination zone
  dm zoned: do not reclaim a random zone into another random zone
  dm zoned: keep polling for idle reclaim after a reclaim pass

 drivers/md/dm-zoned-reclaim.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

-- 
2.47.3