loop: move vfs_fsync() out of loop_update_dio()

If vfs_flush() is called with queue frozen, the queue freeze lock may be
connected with FS internal lock, and lockdep warning can be triggered
because the queue freeze lock is connected with too many global or
sub-system locks.

Fix the warning by moving vfs_fsync() out of loop_update_dio():

- vfs_fsync() is only needed when switching to dio

- only loop_change_fd() and loop_configure() may switch from buffered
IO to direct IO, so call vfs_fsync() directly here. This way is safe
because either loop is in unbound, or new file isn't attached

- for the other two cases of set_status and set_block_size, direct IO
can only become off, so no need to call vfs_fsync()

Cc: Christoph Hellwig <hch@infradead.org>
Reported-by: Kun Hu <huk23@m.fudan.edu.cn>
Reported-by: Jiaji Qin <jjtan24@m.fudan.edu.cn>
Closes: https://lore.kernel.org/linux-block/359BC288-B0B1-4815-9F01-3A349B12E816@m.fudan.edu.cn/T/#u
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250318072955.3893805-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/loop.c

@@ -189,18 +189,12 @@ static bool lo_can_use_dio(struct loop_device *lo)
  */
 static inline void loop_update_dio(struct loop_device *lo)
 {
-	bool dio_in_use = lo->lo_flags & LO_FLAGS_DIRECT_IO;
-
 	lockdep_assert_held(&lo->lo_mutex);
 	WARN_ON_ONCE(lo->lo_state == Lo_bound &&
 		     lo->lo_queue->mq_freeze_depth == 0);
 
 	if ((lo->lo_flags & LO_FLAGS_DIRECT_IO) && !lo_can_use_dio(lo))
 		lo->lo_flags &= ~LO_FLAGS_DIRECT_IO;
-
-	/* flush dirty pages before starting to issue direct I/O */
-	if ((lo->lo_flags & LO_FLAGS_DIRECT_IO) && !dio_in_use)
-		vfs_fsync(lo->lo_backing_file, 0);
 }
 
 /**
@@ -637,6 +631,13 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
 	if (get_loop_size(lo, file) != get_loop_size(lo, old_file))
 		goto out_err;
 
+	/*
+	 * We might switch to direct I/O mode for the loop device, write back
+	 * all dirty data the page cache now that so that the individual I/O
+	 * operations don't have to do that.
+	 */
+	vfs_fsync(file, 0);
+
 	/* and ... switch */
 	disk_force_media_change(lo->lo_disk);
 	memflags = blk_mq_freeze_queue(lo->lo_queue);
@@ -1105,6 +1106,13 @@ static int loop_configure(struct loop_device *lo, blk_mode_t mode,
 	if (error)
 		goto out_unlock;
 
+	/*
+	 * We might switch to direct I/O mode for the loop device, write back
+	 * all dirty data the page cache now that so that the individual I/O
+	 * operations don't have to do that.
+	 */
+	vfs_fsync(file, 0);
+
 	loop_update_dio(lo);
 	loop_sysfs_init(lo);