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

[Christoph Hellwig <hch@infradead.org>]

On Tue, Jul 27, 2010 at 08:01:31PM +0200, Jan Kara wrote:
> > It shouldn't.  Block device nodes are on the bdev filesystems, and
>   Ok, so inode->i_sb->s_bdi will actually point to noop_backing_dev_info
> as set by set_anon_super(). Or am I completely out?

I think you're right.  This seems rather bad if it's indeed true. I'll
quickly verify it using Dave's new tracing once I've built a block
tree kernel.

> > we twist the file->mapping pointer so that all the low-level read/write
> > code always deals with the bdev fs inode, and not the device node
> > filesystem.
>
> Yes, I know this but I fail to see how this influences where </dev/sda's
> inode>->i_sb->s_bdi ends up...

Ok, let's say we write to /dev/sda.  For this we call

	blkdev_aio_write -> __generic_file_aio_write

There we get the inode we operate on as

	struct file *file = iocb->ki_filp;
	struct address_space * mapping = file->f_mapping;
	struct inode    *inode = mapping->host;

Now file->f_mapping gets set up in blkdev_open as:

	filp->f_mapping = bdev->bd_inode->i_mapping;

so it does not point to the inode of the device node, but an inode on
the bdev filesystem.