dm: prepare to support WRITE SAME

Allow targets to opt in to WRITE SAME support by setting
'num_write_same_requests' in the dm_target structure.

A dm device will only advertise WRITE SAME support if all its
targets and all its underlying devices support it.

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

2 files changed, 34 insertions, 1 deletions

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index fa295579003..6be58b69637 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -1414,6 +1414,33 @@ static bool dm_table_all_devices_attribute(struct dm_table *t,
         return 1;
 }
 
+static int device_not_write_same_capable(struct dm_target *ti, struct dm_dev *dev,
+                                         sector_t start, sector_t len, void *data)
+{
+        struct request_queue *q = bdev_get_queue(dev->bdev);
+
+        return q && !q->limits.max_write_same_sectors;
+}
+
+static bool dm_table_supports_write_same(struct dm_table *t)
+{
+        struct dm_target *ti;
+        unsigned i = 0;
+
+        while (i < dm_table_get_num_targets(t)) {
+                ti = dm_table_get_target(t, i++);
+
+                if (!ti->num_write_same_requests)
+                        return false;
+
+                if (!ti->type->iterate_devices ||
+                    !ti->type->iterate_devices(ti, device_not_write_same_capable, NULL))
+                        return false;
+        }
+
+        return true;
+}
+
 void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
                                struct queue_limits *limits)
 {
@@ -1445,7 +1472,8 @@ void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
         else
                 queue_flag_clear_unlocked(QUEUE_FLAG_NONROT, q);
 
-        q->limits.max_write_same_sectors = 0;
+        if (!dm_table_supports_write_same(t))
+                q->limits.max_write_same_sectors = 0;
 
         dm_table_set_integrity(t);
 
diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index 38d27a10aa5..d1f6cd8486f 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -205,6 +205,11 @@ struct dm_target {
          */
         unsigned num_discard_requests;
 
+        /*
+         * The number of WRITE SAME requests that will be submitted to the target.
+         */
+        unsigned num_write_same_requests;
+
         /* target specific data */
         void *private;