loop: remove the racy bd_inode->i_mapping->nrpages asserts

Nothing prevents a file system or userspace opener of the block device
from redirtying the page right afte sync_blockdev returned.  Fortunately
data in the page cache during a block device change is mostly harmless
anyway.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20220330052917.2566582-9-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/loop.c

@@ -1271,15 +1271,6 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 	/* I/O need to be drained during transfer transition */
 	blk_mq_freeze_queue(lo->lo_queue);
 
-	if (size_changed && lo->lo_device->bd_inode->i_mapping->nrpages) {
-		/* If any pages were dirtied after invalidate_bdev(), try again */
-		err = -EAGAIN;
-		pr_warn("%s: loop%d (%s) still has dirty pages (nrpages=%lu)\n",
-			__func__, lo->lo_number, lo->lo_file_name,
-			lo->lo_device->bd_inode->i_mapping->nrpages);
-		goto out_unfreeze;
-	}
-
 	prev_lo_flags = lo->lo_flags;
 
 	err = loop_set_status_from_info(lo, info);
@@ -1490,21 +1481,10 @@ static int loop_set_block_size(struct loop_device *lo, unsigned long arg)
 	invalidate_bdev(lo->lo_device);
 
 	blk_mq_freeze_queue(lo->lo_queue);
-
-	/* invalidate_bdev should have truncated all the pages */
-	if (lo->lo_device->bd_inode->i_mapping->nrpages) {
-		err = -EAGAIN;
-		pr_warn("%s: loop%d (%s) still has dirty pages (nrpages=%lu)\n",
-			__func__, lo->lo_number, lo->lo_file_name,
-			lo->lo_device->bd_inode->i_mapping->nrpages);
-		goto out_unfreeze;
-	}
-
 	blk_queue_logical_block_size(lo->lo_queue, arg);
 	blk_queue_physical_block_size(lo->lo_queue, arg);
 	blk_queue_io_min(lo->lo_queue, arg);
 	loop_update_dio(lo);
-out_unfreeze:
 	blk_mq_unfreeze_queue(lo->lo_queue);
 
 	return err;