dm space maps: don't reset space map allocation cursor when committing

Current commit code resets the place where the search for free blocks
will begin back to the start of the metadata device. There are a couple
of repercussions to this:

- The first allocation after the commit is likely to take longer than
normal as it searches for a free block in an area that is likely to
have very few free blocks (if any).

- Any free blocks it finds will have been recently freed. Reusing them
means we have fewer old copies of the metadata to aid recovery from
hardware error.

Fix these issues by leaving the cursor alone, only resetting when the
search hits the end of the metadata device.

Signed-off-by: Joe Thornber <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

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

···
 	 * Any block we allocate has to be free in both the old and current ll.
 	 */
 	r = sm_ll_find_common_free_block(&smd->old_ll, &smd->ll, smd->begin, smd->ll.nr_blocks, b);
+	if (r == -ENOSPC) {
+		/*
+		 * There's no free block between smd->begin and the end of the metadata device.
+		 * We search before smd->begin in case something has been freed.
+		 */
+		r = sm_ll_find_common_free_block(&smd->old_ll, &smd->ll, 0, smd->begin, b);
+	}
+
 	if (r)
 		return r;
 
···
 		return r;
 
 	memcpy(&smd->old_ll, &smd->ll, sizeof(smd->old_ll));
-	smd->begin = 0;
 	smd->nr_allocated_this_transaction = 0;
 
 	return 0;

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

···
 	 * Any block we allocate has to be free in both the old and current ll.
 	 */
 	r = sm_ll_find_common_free_block(&smm->old_ll, &smm->ll, smm->begin, smm->ll.nr_blocks, b);
+	if (r == -ENOSPC) {
+		/*
+		 * There's no free block between smm->begin and the end of the metadata device.
+		 * We search before smm->begin in case something has been freed.
+		 */
+		r = sm_ll_find_common_free_block(&smm->old_ll, &smm->ll, 0, smm->begin, b);
+	}
+
 	if (r)
 		return r;
 
···
 		return r;
 
 	memcpy(&smm->old_ll, &smm->ll, sizeof(smm->old_ll));
-	smm->begin = 0;
 	smm->allocated_this_transaction = 0;
 
 	return 0;