[RFC PATCH 54/68] vfs: Add a single-or-reconfig keying to vfs_get_super()

[David Howells <dhowells@redhat.com>]

Add an additional keying mode to vfs_get_super() to indicate that only a
single superblock should exist in the system, and that, if it does, further
mounts should invoke reconfiguration upon it.

This allows mount_single() to be replaced.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 fs/super.c                 |   24 +++++++++++++++++-------
 include/linux/fs_context.h |    1 +
 2 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 6cb57ad31e6f..60c9caeae3c7 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1175,9 +1175,11 @@ int vfs_get_super(struct fs_context *fc,
 {
 	int (*test)(struct super_block *, struct fs_context *);
 	struct super_block *sb;
+	int err;
 
 	switch (keying) {
 	case vfs_get_single_super:
+	case vfs_get_single_reconf_super:
 		test = test_single_super;
 		break;
 	case vfs_get_keyed_super:
@@ -1195,18 +1197,26 @@ int vfs_get_super(struct fs_context *fc,
 		return PTR_ERR(sb);
 
 	if (!sb->s_root) {
-		int err = fill_super(sb, fc);
-		if (err) {
-			deactivate_locked_super(sb);
-			return err;
-		}
+		err = fill_super(sb, fc);
+		if (err)
+			goto error;
 
 		sb->s_flags |= SB_ACTIVE;
+		fc->root = dget(sb->s_root);
+	} else {
+		fc->root = dget(sb->s_root);
+		if (keying == vfs_get_single_reconf_super) {
+			err = reconfigure_super(fc);
+			if (err < 0)
+				goto error;
+		}
 	}
 
-	BUG_ON(fc->root);
-	fc->root = dget(sb->s_root);
 	return 0;
+
+error:
+	deactivate_locked_super(sb);
+	return err;
 }
 EXPORT_SYMBOL(vfs_get_super);
 
diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
index 7c7eefb35cc3..3b1da9b2db91 100644
--- a/include/linux/fs_context.h
+++ b/include/linux/fs_context.h
@@ -138,6 +138,7 @@ extern int vfs_init_pseudo_fs_context(struct fs_context *fc,
  */
 enum vfs_get_super_keying {
 	vfs_get_single_super,	/* Only one such superblock may exist */
+	vfs_get_single_reconf_super, /* As above, but reconfigure if it exists */
 	vfs_get_keyed_super,	/* Superblocks with different s_fs_info keys may exist */
 	vfs_get_independent_super, /* Multiple independent superblocks may exist */
 };