Re: [patch] fs: fix deadlocks in writeback_if_idle

[Chris Mason <chris.mason@oracle.com>]

Excerpts from Andrew Morton's message of 2010-11-24 17:47:40 -0500:
> On Tue, 23 Nov 2010 07:34:07 -0500
> Chris Mason <chris.mason@oracle.com> wrote:
> 
> > For btrfs there's only one bdi per SB, but for most everyone else a disk
> > with a bunch of partitions is going to have multiple filesystems on the
> > same bdi.
> 
> um, please explain why that wasn't idiotic?  The BDI is a
> representation of a backing device and it's *supposed* to provide
> visibility into what's happening against other partitions on the same
> device.  Creating a BDI per SB (it didn't even occur to me to think
> that a filesystem was even able to do this) breaks that.
> 

We don't really have visibility into the other partitions, we all just
pretend they aren't there (this patch being the most recent example).
Yes, it does help prevent N flushers seeking around on the drive but it
does cause problems too.

How is the btrfs one-bdi-per-super different from device mapper's one
bdi per logical volume?  We're all idiots together I suppose.

As for having multiple bdis per FS, that was always a long term goal of
mine when Jens was setting up the new flushers.  I didn't want to
confuse the initial implementation with it though.

-chris