fs: add a method to shut down the file system

Add a new ->shutdown super operation that can be used to tell the file
system to shut down, and call it from newly created holder ops when the
block device under a file system shuts down.

This only covers the main block device for "simple" file systems using
get_tree_bdev / mount_bdev.  File systems their own get_tree method
or opening additional devices will need to set up their own
blk_holder_ops.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Acked-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Link: https://lore.kernel.org/r/20230601094459.1350643-12-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/super.c

@@ -1206,6 +1206,22 @@ int get_tree_keyed(struct fs_context *fc,
 EXPORT_SYMBOL(get_tree_keyed);
 
 #ifdef CONFIG_BLOCK
+static void fs_mark_dead(struct block_device *bdev)
+{
+	struct super_block *sb;
+
+	sb = get_super(bdev);
+	if (!sb)
+		return;
+
+	if (sb->s_op->shutdown)
+		sb->s_op->shutdown(sb);
+	drop_super(sb);
+}
+
+static const struct blk_holder_ops fs_holder_ops = {
+	.mark_dead		= fs_mark_dead,
+};
 
 static int set_bdev_super(struct super_block *s, void *data)
 {
@@ -1248,7 +1264,8 @@ int get_tree_bdev(struct fs_context *fc,
 	if (!fc->source)
 		return invalf(fc, "No source specified");
 
-	bdev = blkdev_get_by_path(fc->source, mode, fc->fs_type, NULL);
+	bdev = blkdev_get_by_path(fc->source, mode, fc->fs_type,
+				  &fs_holder_ops);
 	if (IS_ERR(bdev)) {
 		errorf(fc, "%s: Can't open blockdev", fc->source);
 		return PTR_ERR(bdev);
@@ -1333,7 +1350,7 @@ struct dentry *mount_bdev(struct file_system_type *fs_type,
 	if (!(flags & SB_RDONLY))
 		mode |= FMODE_WRITE;
 
-	bdev = blkdev_get_by_path(dev_name, mode, fs_type, NULL);
+	bdev = blkdev_get_by_path(dev_name, mode, fs_type, &fs_holder_ops);
 	if (IS_ERR(bdev))
 		return ERR_CAST(bdev);
 

include/linux/fs.h

@@ -1932,6 +1932,7 @@ struct super_operations {
 				  struct shrink_control *);
 	long (*free_cached_objects)(struct super_block *,
 				    struct shrink_control *);
+	void (*shutdown)(struct super_block *sb);
 };
 
 /*