[PATCH v7 2/2] dm zoned: do not reclaim a random zone into another random zone

[Kyrylo Biryuk <illofspeed@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 kept reading and writing while the free zone counters
in "dmsetup status" did not change.

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

This depends on "dm zoned: back off when reclaim cannot make progress":
without it, dmz_reclaim_work() would retry the resulting -ENOSPC at
once, and run dmz_check_dev(), for as long as the target stays idle.

Tested on 7.4-rc1 together with the previous patch, on a two-device
target (a 512 MiB RAM-backed regular cache device plus a 1 GiB
scsi_debug host-managed drive) filled until no sequential zone was
free, then idle for 60 seconds: the stock module made 877
random-to-random copies; with this series it made none, and each pass
failed with -ENOSPC once per DMZ_IDLE_PERIOD without a device check.

Fixes: c5c788595292 ("dm zoned: start reclaim with sequential zones")
Cc: stable@vger.kernel.org
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@kernel.org>
Signed-off-by: Kyrylo Biryuk <illofspeed@gmail.com>
---
 drivers/md/dm-zoned-reclaim.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-zoned-reclaim.c b/drivers/md/dm-zoned-reclaim.c
index 077a8a50197..152cb878811 100644
--- a/drivers/md/dm-zoned-reclaim.c
+++ b/drivers/md/dm-zoned-reclaim.c
@@ -288,7 +288,13 @@ 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)) {
+	/*
+	 * If no free sequential zone is available, a cache zone can still be
+	 * moved into a free random zone of the zoned device, which frees cache
+	 * space. Do not do this for a random zone: moving it into another
+	 * random zone frees nothing and, on an idle target, repeats forever.
+	 */
+	if (!szone && alloc_flags == DMZ_ALLOC_SEQ && dmz_is_cache(dzone)) {
 		alloc_flags = DMZ_ALLOC_RND;
 		goto again;
 	}
-- 
2.43.0