bcachefs: Fix "trying to move an extent, but nr_replicas=0"

data_update_init() does a bunch of complicated stuff to decide how many
replicas to add, since we only want to increase an extent's durability
on an explicit rereplicate, but extent pointers may be on devices with
different durability settings.

There was a corner case when evacuating a device that had been set to
durability=0 after data had been written to it, and extents on that
device had already been rereplicated - then evacuate only needs to drop
pointers on that device, not move them.

So the assert for !m->op.nr_replicas was spurious; this was a perfectly
legitimate case that needed to be handled.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/data_update.c

@@ -695,16 +695,6 @@ int bch2_data_update_init(struct btree_trans *trans,
 	 * Increasing replication is an explicit operation triggered by
 	 * rereplicate, currently, so that users don't get an unexpected -ENOSPC
 	 */
-	if (!(m->data_opts.write_flags & BCH_WRITE_CACHED) &&
-	    !durability_required) {
-		m->data_opts.kill_ptrs |= m->data_opts.rewrite_ptrs;
-		m->data_opts.rewrite_ptrs = 0;
-		/* if iter == NULL, it's just a promote */
-		if (iter)
-			ret = bch2_extent_drop_ptrs(trans, iter, k, m->data_opts);
-		goto out;
-	}
-
 	m->op.nr_replicas = min(durability_removing, durability_required) +
 		m->data_opts.extra_replicas;
 
@@ -716,17 +706,22 @@ int bch2_data_update_init(struct btree_trans *trans,
 	if (!(durability_have + durability_removing))
 		m->op.nr_replicas = max((unsigned) m->op.nr_replicas, 1);
 
-	if (!m->op.nr_replicas) {
-		struct printbuf buf = PRINTBUF;
+	m->op.nr_replicas_required = m->op.nr_replicas;
 
-		bch2_data_update_to_text(&buf, m);
-		WARN(1, "trying to move an extent, but nr_replicas=0\n%s", buf.buf);
-		printbuf_exit(&buf);
+	/*
+	 * It might turn out that we don't need any new replicas, if the
+	 * replicas or durability settings have been changed since the extent
+	 * was written:
+	 */
+	if (!m->op.nr_replicas) {
+		m->data_opts.kill_ptrs |= m->data_opts.rewrite_ptrs;
+		m->data_opts.rewrite_ptrs = 0;
+		/* if iter == NULL, it's just a promote */
+		if (iter)
+			ret = bch2_extent_drop_ptrs(trans, iter, k, m->data_opts);
 		goto out;
 	}
 
-	m->op.nr_replicas_required = m->op.nr_replicas;
-
 	if (reserve_sectors) {
 		ret = bch2_disk_reservation_add(c, &m->op.res, reserve_sectors,
 				m->data_opts.extra_replicas