loop: raise media_change event

Make the loop device raise a DISK_MEDIA_CHANGE event on attach or detach.

	# udevadm monitor -up |grep -e DISK_MEDIA_CHANGE -e DEVNAME &

	# losetup -f zero
	[    7.454235] loop0: detected capacity change from 0 to 16384
	DISK_MEDIA_CHANGE=1
	DEVNAME=/dev/loop0
	DEVNAME=/dev/loop0
	DEVNAME=/dev/loop0

	# losetup -f zero
	[   10.205245] loop1: detected capacity change from 0 to 16384
	DISK_MEDIA_CHANGE=1
	DEVNAME=/dev/loop1
	DEVNAME=/dev/loop1
	DEVNAME=/dev/loop1

	# losetup -f zero2
	[   13.532368] loop2: detected capacity change from 0 to 40960
	DISK_MEDIA_CHANGE=1
	DEVNAME=/dev/loop2
	DEVNAME=/dev/loop2

	# losetup -D
	DEVNAME=/dev/loop1
	DISK_MEDIA_CHANGE=1
	DEVNAME=/dev/loop1
	DEVNAME=/dev/loop2
	DISK_MEDIA_CHANGE=1
	DEVNAME=/dev/loop2
	DEVNAME=/dev/loop0
	DISK_MEDIA_CHANGE=1
	DEVNAME=/dev/loop0

Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Luca Boccassi <bluca@debian.org>
Link: https://lore.kernel.org/r/20210712230530.29323-7-mcroce@linux.microsoft.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/loop.c

@@ -774,6 +774,7 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
 		goto out_err;
 
 	/* and ... switch */
+	disk_force_media_change(lo->lo_disk, DISK_EVENT_MEDIA_CHANGE);
 	blk_mq_freeze_queue(lo->lo_queue);
 	mapping_set_gfp_mask(old_file->f_mapping, lo->old_gfp_mask);
 	lo->lo_backing_file = file;
@@ -1257,6 +1258,7 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
 		goto out_unlock;
 	}
 
+	disk_force_media_change(lo->lo_disk, DISK_EVENT_MEDIA_CHANGE);
 	set_disk_ro(lo->lo_disk, (lo->lo_flags & LO_FLAGS_READ_ONLY) != 0);
 
 	INIT_WORK(&lo->rootcg_work, loop_rootcg_workfn);
@@ -1410,6 +1412,7 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
 
 	partscan = lo->lo_flags & LO_FLAGS_PARTSCAN && bdev;
 	lo_number = lo->lo_number;
+	disk_force_media_change(lo->lo_disk, DISK_EVENT_MEDIA_CHANGE);
 out_unlock:
 	mutex_unlock(&lo->lo_mutex);
 	if (partscan) {
@@ -2386,6 +2389,8 @@ static int loop_add(int i)
 	disk->fops		= &lo_fops;
 	disk->private_data	= lo;
 	disk->queue		= lo->lo_queue;
+	disk->events		= DISK_EVENT_MEDIA_CHANGE;
+	disk->event_flags	= DISK_EVENT_FLAG_UEVENT;
 	sprintf(disk->disk_name, "loop%d", i);
 	add_disk(disk);
 	mutex_unlock(&loop_ctl_mutex);