Re: [PATCH 1/3] bdi: Use parent filesystem BDI for inodes not capable of writeback

[Christoph Hellwig <hch@infradead.org>]

Given that we only need the per-mapping one for the bdev fs what about
doing:

/*
 * Return the writeback-relevant backing device for this inode.
 *
 * For a normal filesystem this must always be the bdi hanging off the
 * superblock, given that we only expect one bdi per filesystems in
 * the per-superblock sync functions.  But the block device special
 * filesystem requires a quick given that it contains the internal
 * I/O inodes for block devices on a single superblock.  This works
 * because the block deevice filesystem inodes are never user visible
 * and we will never do a per-superblock sync on it.
 */
static struct backing_dev_info *inode_to_bdi(struct inode *inode)
{
	if (inode->i_sb == blockdev_superblock)
		return inode->i_mapping->backing_dev_info;
	return inode->i_sb->s_bdi;
}

that also avoids the need for doing the bdi capabilities audit ASAP.