dm space map common: memcpy the disk root to ensure it's arch aligned

The metadata_space_map_root passed to sm_ll_open_metadata() may or may
not be arch aligned, use memcpy to ensure it is.  This is not a fast
path so the extra memcpy doesn't hurt us.

Long-term it'd be better to use the kernel's alignment infrastructure to
remove the memcpy()s that are littered across persistent-data (btree,
array, space-maps, etc).
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

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

@@ -626,13 +626,19 @@ int sm_ll_open_metadata(struct ll_disk *ll, struct dm_transaction_manager *tm,
 			void *root_le, size_t len)
 {
 	int r;
-	struct disk_sm_root *smr = root_le;
+	struct disk_sm_root smr;
 
 	if (len < sizeof(struct disk_sm_root)) {
 		DMERR("sm_metadata root too small");
 		return -ENOMEM;
 	}
 
+	/*
+	 * We don't know the alignment of the root_le buffer, so need to
+	 * copy into a new structure.
+	 */
+	memcpy(&smr, root_le, sizeof(smr));
+
 	r = sm_ll_init(ll, tm);
 	if (r < 0)
 		return r;
@@ -644,10 +650,10 @@ int sm_ll_open_metadata(struct ll_disk *ll, struct dm_transaction_manager *tm,
 	ll->max_entries = metadata_ll_max_entries;
 	ll->commit = metadata_ll_commit;
 
-	ll->nr_blocks = le64_to_cpu(smr->nr_blocks);
-	ll->nr_allocated = le64_to_cpu(smr->nr_allocated);
-	ll->bitmap_root = le64_to_cpu(smr->bitmap_root);
-	ll->ref_count_root = le64_to_cpu(smr->ref_count_root);
+	ll->nr_blocks = le64_to_cpu(smr.nr_blocks);
+	ll->nr_allocated = le64_to_cpu(smr.nr_allocated);
+	ll->bitmap_root = le64_to_cpu(smr.bitmap_root);
+	ll->ref_count_root = le64_to_cpu(smr.ref_count_root);
 
 	return ll->open_index(ll);
 }