dm thin: do not expose non-zero discard limits if discards disabled

Fix issue where the block layer would stack the discard limits of the
pool's data device even if the "ignore_discard" pool feature was
specified.

The pool and thin device(s) still had discards disabled because the
QUEUE_FLAG_DISCARD request_queue flag wasn't set.  But to avoid user
confusion when "ignore_discard" is used: both the pool device and the
thin device(s) have zeroes for all discard limits.

Also, always set discard_zeroes_data_unsupported in targets because they
should never advertise the 'discard_zeroes_data' capability (even if the
pool's data device supports it).

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>

1 files changed, 11 insertions, 3 deletions

diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index ed063427d676..2c0cf511ec23 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -2095,6 +2095,7 @@ static int pool_ctr(struct dm_target *ti, unsigned argc, char **argv)
          * them down to the data device.  The thin device's discard
          * processing will cause mappings to be removed from the btree.
          */
+        ti->discard_zeroes_data_unsupported = true;
         if (pf.discard_enabled && pf.discard_passdown) {
                 ti->num_discard_bios = 1;
 
@@ -2104,7 +2105,6 @@ static int pool_ctr(struct dm_target *ti, unsigned argc, char **argv)
                  * thin devices' discard limits consistent).
                  */
                 ti->discards_supported = true;
-                ti->discard_zeroes_data_unsupported = true;
         }
         ti->private = pt;
 
@@ -2689,8 +2689,16 @@ static void pool_io_hints(struct dm_target *ti, struct queue_limits *limits)
          * They get transferred to the live pool in bind_control_target()
          * called from pool_preresume().
          */
-        if (!pt->adjusted_pf.discard_enabled)
+        if (!pt->adjusted_pf.discard_enabled) {
+                /*
+                 * Must explicitly disallow stacking discard limits otherwise the
+                 * block layer will stack them if pool's data device has support.
+                 * QUEUE_FLAG_DISCARD wouldn't be set but there is no way for the
+                 * user to see that, so make sure to set all discard limits to 0.
+                 */
+                limits->discard_granularity = 0;
                 return;
+        }
 
         disable_passdown_if_not_supported(pt);
 
@@ -2826,10 +2834,10 @@ static int thin_ctr(struct dm_target *ti, unsigned argc, char **argv)
         ti->per_bio_data_size = sizeof(struct dm_thin_endio_hook);
 
         /* In case the pool supports discards, pass them on. */
+        ti->discard_zeroes_data_unsupported = true;
         if (tc->pool->pf.discard_enabled) {
                 ti->discards_supported = true;
                 ti->num_discard_bios = 1;
-                ti->discard_zeroes_data_unsupported = true;
                 /* Discard bios must be split on a block boundary */
                 ti->split_discard_bios = true;
         }