bcachefs: Ensure journal doesn't get stuck in nochanges mode

This tweaks the journal code to always act as if there's space available
in nochanges mode, when we're not going to be doing any writes. This
helps in recovering filesystems that won't mount because they need
journal replay and the journal has gotten stuck.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/journal.h

@@ -446,6 +446,7 @@ static inline int bch2_journal_preres_get_fast(struct journal *j,
 		ret = 0;
 
 		if ((flags & JOURNAL_RES_GET_RESERVED) ||
+		    test_bit(JOURNAL_NOCHANGES, &j->flags) ||
 		    new.reserved + d < new.remaining) {
 			new.reserved += d;
 			ret = 1;

fs/bcachefs/journal_io.c

@@ -1512,7 +1512,7 @@ void bch2_journal_write(struct closure *cl)
 
 	w->devs_written = bch2_bkey_devs(bkey_i_to_s_c(&w->key));
 
-	if (c->opts.nochanges)
+	if (test_bit(JOURNAL_NOCHANGES, &j->flags))
 		goto no_io;
 
 	for_each_rw_member(ca, c, i)

fs/bcachefs/journal_reclaim.c

@@ -34,8 +34,10 @@ unsigned bch2_journal_dev_buckets_available(struct journal *j,
 					    struct journal_device *ja,
 					    enum journal_space_from from)
 {
-	unsigned available = (journal_space_from(ja, from) -
-			      ja->cur_idx - 1 + ja->nr) % ja->nr;
+	unsigned available = !test_bit(JOURNAL_NOCHANGES, &j->flags)
+		? ((journal_space_from(ja, from) -
+		    ja->cur_idx - 1 + ja->nr) % ja->nr)
+		: ja->nr;
 
 	/*
 	 * Don't use the last bucket unless writing the new last_seq

fs/bcachefs/journal_types.h

@@ -154,6 +154,7 @@ enum {
 	JOURNAL_NEED_WRITE,
 	JOURNAL_MAY_GET_UNRESERVED,
 	JOURNAL_MAY_SKIP_FLUSH,
+	JOURNAL_NOCHANGES,
 };
 
 /* Embedded in struct bch_fs */

fs/bcachefs/super.c

@@ -801,6 +801,9 @@ static struct bch_fs *bch2_fs_alloc(struct bch_sb *sb, struct bch_opts opts)
 	    bch2_fs_fsio_init(c))
 		goto err;
 
+	if (c->opts.nochanges)
+		set_bit(JOURNAL_NOCHANGES, &c->journal.flags);
+
 	mi = bch2_sb_get_members(c->disk_sb.sb);
 	for (i = 0; i < c->sb.nr_devices; i++)
 		if (bch2_dev_exists(c->disk_sb.sb, mi, i) &&