dm zoned: Fix zone reclaim trigger

Only triggering reclaim based on the percentage of unmapped cache
zones can fail to detect cases where reclaim is needed, e.g. if the
target has only 2 or 3 cache zones and only one unmapped cache zone,
the percentage of free cache zones is higher than
DMZ_RECLAIM_LOW_UNMAP_ZONES (30%) and reclaim does not trigger.

This problem, combined with the fact that dmz_schedule_reclaim() is
called from dmz_handle_bio() without the map lock held, leads to a
race between zone allocation and dmz_should_reclaim() result.
Depending on the workload applied, this race can lead to the write
path waiting forever for a free zone without reclaim being triggered.

Fix this by moving dmz_schedule_reclaim() inside dmz_alloc_zone()
under the map lock. This results in checking the need for zone reclaim
whenever a new data or buffer zone needs to be allocated.

Also fix dmz_reclaim_percentage() to always return 0 if the number of
unmapped cache (or random) zones is less than or equal to 1.

Suggested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/drivers/md/dm-zoned-reclaim.c b/drivers/md/dm-zoned-reclaim.c
index 7e0cc2d732cf..9c0ecc9568a4 100644
--- a/drivers/md/dm-zoned-reclaim.c
+++ b/drivers/md/dm-zoned-reclaim.c
@@ -456,6 +456,8 @@ static unsigned int dmz_reclaim_percentage(struct dmz_reclaim *zrc)
 		nr_zones = dmz_nr_rnd_zones(zmd, zrc->dev_idx);
 		nr_unmap = dmz_nr_unmap_rnd_zones(zmd, zrc->dev_idx);
 	}
+	if (nr_unmap <= 1)
+		return 0;
 	return nr_unmap * 100 / nr_zones;
 }