bcachefs: Fix nochanges/read_only interaction

nochanges means "we cannot issue writes at all"; it's possible to go
into a pseudo read-write mode where we pin dirty metadata in memory,
which is used for fsck in dry run mode and doing journal replay on a
read only mount, but we do not want to allow an actual read-write mount
in nochanges mode.

But we do always want to allow early read-write, during recovery - this
patch clarifies that.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs.c

@@ -1630,12 +1630,12 @@ static int bch2_remount(struct super_block *sb, int *flags, char *data)
 	struct bch_opts opts = bch2_opts_empty();
 	int ret;
 
-	opt_set(opts, read_only, (*flags & SB_RDONLY) != 0);
-
 	ret = bch2_parse_mount_opts(c, &opts, data);
 	if (ret)
 		goto err;
 
+	opt_set(opts, read_only, (*flags & SB_RDONLY) != 0);
+
 	if (opts.read_only != c->opts.read_only) {
 		down_write(&c->state_lock);
 

fs/bcachefs/opts.h

@@ -389,7 +389,7 @@ enum fsck_err_opts {
 	  BCH2_NO_SB_OPT,		BCH_SB_SECTOR,			\
 	  "offset",	"Sector offset of superblock")			\
 	x(read_only,			u8,				\
-	  OPT_FS,							\
+	  OPT_FS|OPT_MOUNT,						\
 	  OPT_BOOL(),							\
 	  BCH2_NO_SB_OPT,		false,				\
 	  NULL,		NULL)						\

fs/bcachefs/recovery.c

@@ -1076,7 +1076,9 @@ int bch2_fs_recovery(struct bch_fs *c)
 		bch2_journal_keys_put_initial(c);
 	kfree(clean);
 
-	if (!ret && test_bit(BCH_FS_need_delete_dead_snapshots, &c->flags)) {
+	if (!ret &&
+	    test_bit(BCH_FS_need_delete_dead_snapshots, &c->flags) &&
+	    !c->opts.nochanges) {
 		bch2_fs_read_write_early(c);
 		bch2_delete_dead_snapshots_async(c);
 	}

fs/bcachefs/super.c

@@ -433,16 +433,6 @@ static int __bch2_fs_read_write(struct bch_fs *c, bool early)
 	if (test_bit(BCH_FS_rw, &c->flags))
 		return 0;
 
-	if (c->opts.norecovery)
-		return -BCH_ERR_erofs_norecovery;
-
-	/*
-	 * nochanges is used for fsck -n mode - we have to allow going rw
-	 * during recovery for that to work:
-	 */
-	if (c->opts.nochanges && (!early || c->opts.read_only))
-		return -BCH_ERR_erofs_nochanges;
-
 	bch_info(c, "going read-write");
 
 	ret = bch2_sb_members_v2_init(c);
@@ -510,6 +500,12 @@ static int __bch2_fs_read_write(struct bch_fs *c, bool early)
 
 int bch2_fs_read_write(struct bch_fs *c)
 {
+	if (c->opts.norecovery)
+		return -BCH_ERR_erofs_norecovery;
+
+	if (c->opts.nochanges)
+		return -BCH_ERR_erofs_nochanges;
+
 	return __bch2_fs_read_write(c, false);
 }
 
@@ -1033,7 +1029,7 @@ int bch2_fs_start(struct bch_fs *c)
 
 	set_bit(BCH_FS_started, &c->flags);
 
-	if (c->opts.read_only || c->opts.nochanges) {
+	if (c->opts.read_only) {
 		bch2_fs_read_only(c);
 	} else {
 		ret = !test_bit(BCH_FS_rw, &c->flags)
@@ -1946,6 +1942,11 @@ struct bch_fs *bch2_fs_open(char * const *devices, unsigned nr_devices,
 		BUG_ON(darray_push(&sbs, sb));
 	}
 
+	if (opts.nochanges && !opts.read_only) {
+		ret = -BCH_ERR_erofs_nochanges;
+		goto err_print;
+	}
+
 	darray_for_each(sbs, sb)
 		if (!best || le64_to_cpu(sb->sb->seq) > le64_to_cpu(best->sb->seq))
 			best = sb;