[PATCH 2/3] dm zoned: do not reclaim a random zone into another random zone

[illofspeed <volvo.mail@gmail.com>]

dmz_reclaim_rnd_data() falls back to a random destination zone when no
free sequential zone is available and the target has cache zones. When
the target is idle and the cache zone list is empty,
dmz_get_rnd_zone_for_reclaim() selects random zones of the zoned device
as well, so a random zone is then copied into another random zone. That
frees nothing: the next pass picks the next random zone and copies it
again, for as long as the target stays idle.

Seen on a two-device target (256 GiB regular cache device plus a 27 TB
host-managed drive) used as an md RAID5 member: after a full rebuild
every chunk was mapped and no sequential zone was free, and the idle
drive read and wrote about 72 MB/s each without pause for hours while
the free zone counters in "dmsetup status" did not change. Reproduced on
emulated host-managed drives (tcmu-runner ZBC handler): 340 MB/s of
copying on an idle target, 0 MB/s with this patch.

Only allow the random-zone fallback when the zone being reclaimed is a
cache zone: moving it to the zoned device still frees cache space.

Fixes: c5c788595292 ("dm zoned: start reclaim with sequential zones")
Assisted-by: LLM
Signed-off-by: illofspeed <volvo.mail@gmail.com>
---
 drivers/md/dm-zoned-reclaim.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-zoned-reclaim.c b/drivers/md/dm-zoned-reclaim.c
index 315f0da99f3..2bcc163d209 100644
--- a/drivers/md/dm-zoned-reclaim.c
+++ b/drivers/md/dm-zoned-reclaim.c
@@ -288,7 +288,14 @@ static int dmz_reclaim_rnd_data(struct dmz_reclaim *zrc, struct dm_zone *dzone)
 again:
 	szone = dmz_alloc_zone(zmd, zrc->dev_idx,
 			       alloc_flags | DMZ_ALLOC_RECLAIM);
-	if (!szone && alloc_flags == DMZ_ALLOC_SEQ && dmz_nr_cache_zones(zmd)) {
+	/*
+	 * Without a free sequential zone, moving a cache zone into a random
+	 * zone of the zoned device still frees cache space. Moving a random
+	 * zone into another random zone gains nothing, and when the target
+	 * is idle it repeats forever. Only fall back for cache zones.
+	 */
+	if (!szone && alloc_flags == DMZ_ALLOC_SEQ && dmz_nr_cache_zones(zmd) &&
+	    dmz_is_cache(dzone)) {
 		alloc_flags = DMZ_ALLOC_RND;
 		goto again;
 	}
-- 
2.43.0