dm persistent data: only commit space map if index changed

Introduce bitmap_index_changed to track whether or not the index changed
then only commit a space map if it did.
Signed-off-by: NJoe Thornber <ejt@redhat.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>

drivers/md/persistent-data/dm-space-map-common.c

@@ -224,6 +224,7 @@ static int sm_ll_init(struct ll_disk *ll, struct dm_transaction_manager *tm)
 	ll->nr_blocks = 0;
 	ll->bitmap_root = 0;
 	ll->ref_count_root = 0;
+	ll->bitmap_index_changed = false;
 
 	return 0;
 }
@@ -476,7 +477,15 @@ int sm_ll_dec(struct ll_disk *ll, dm_block_t b, enum allocation_event *ev)
 
 int sm_ll_commit(struct ll_disk *ll)
 {
-	return ll->commit(ll);
+	int r = 0;
+
+	if (ll->bitmap_index_changed) {
+		r = ll->commit(ll);
+		if (!r)
+			ll->bitmap_index_changed = false;
+	}
+
+	return r;
 }
 
 /*----------------------------------------------------------------*/
@@ -491,6 +500,7 @@ static int metadata_ll_load_ie(struct ll_disk *ll, dm_block_t index,
 static int metadata_ll_save_ie(struct ll_disk *ll, dm_block_t index,
 			       struct disk_index_entry *ie)
 {
+	ll->bitmap_index_changed = true;
 	memcpy(ll->mi_le.index + index, ie, sizeof(*ie));
 	return 0;
 }

drivers/md/persistent-data/dm-space-map-common.h

@@ -78,6 +78,7 @@ struct ll_disk {
 	open_index_fn open_index;
 	max_index_entries_fn max_entries;
 	commit_fn commit;
+	bool bitmap_index_changed:1;
 };
 
 struct disk_sm_root {