dm table: fix discard support

Remove 'discards_supported' from the dm_table structure.  The same
information can be easily discovered from the table's target(s) in
dm_table_supports_discards().

Before this fix dm_table_supports_discards() would skip checking the
individual targets' 'discards_supported' flag if any one target in the
table didn't set num_discard_requests > 0.  Now the per-target
'discards_supported' flag is effective at insuring the final DM device
advertises discard support.  But, to be clear, targets that don't
support discards (!num_discard_requests) will not receive discard
requests.

Also DMWARN if a target sets 'discards_supported' override but forgets
to set 'num_discard_requests'.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 7 insertions, 8 deletions

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index bfe9c2333cea..3909fa259f55 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -54,7 +54,6 @@ struct dm_table {
         sector_t *highs;
         struct dm_target *targets;
 
-        unsigned discards_supported:1;
         unsigned integrity_supported:1;
 
         /*
@@ -209,7 +208,6 @@ int dm_table_create(struct dm_table **result, fmode_t mode,
         INIT_LIST_HEAD(&t->devices);
         INIT_LIST_HEAD(&t->target_callbacks);
         atomic_set(&t->holders, 0);
-        t->discards_supported = 1;
 
         if (!num_targets)
                 num_targets = KEYS_PER_NODE;
@@ -791,8 +789,9 @@ int dm_table_add_target(struct dm_table *t, const char *type,
 
         t->highs[t->num_targets++] = tgt->begin + tgt->len - 1;
 
-        if (!tgt->num_discard_requests)
-                t->discards_supported = 0;
+        if (!tgt->num_discard_requests && tgt->discards_supported)
+                DMWARN("%s: %s: ignoring discards_supported because num_discard_requests is zero.",
+                       dm_device_name(t->md), type);
 
         return 0;
 
@@ -1359,19 +1358,19 @@ bool dm_table_supports_discards(struct dm_table *t)
         struct dm_target *ti;
         unsigned i = 0;
 
-        if (!t->discards_supported)
-                return 0;
-
         /*
          * Unless any target used by the table set discards_supported,
          * require at least one underlying device to support discards.
          * t->devices includes internal dm devices such as mirror logs
          * so we need to use iterate_devices here, which targets
-         * supporting discard must provide.
+         * supporting discard selectively must provide.
          */
         while (i < dm_table_get_num_targets(t)) {
                 ti = dm_table_get_target(t, i++);
 
+                if (!ti->num_discard_requests)
+                        continue;
+
                 if (ti->discards_supported)
                         return 1;