Re: Dirtiable inode bdi default != sb bdi btrfs

[Christoph Hellwig <hch@lst.de>]

On Wed, Sep 29, 2010 at 02:18:08PM +0200, Jan Kara wrote:
> On Wed 29-09-10 10:19:36, Christoph Hellwig wrote:
> > ---
> > From: Christoph Hellwig <hch@lst.de>
> > Subject: [PATCH] writeback: always use sb->s_bdi for writeback purposes
> > 
> ...
> > The one exception for now is the block device filesystem which really
> > wants different writeback contexts for it's different (internal) inodes
> > to handle the writeout more efficiently.  For now we do this with
> > a hack in fs-writeback.c because we're so late in the cycle, but in
> > the future I plan to replace this with a superblock method that allows
> > for multiple writeback contexts per filesystem.
>   Another exception I know about is mtd_inodefs filesystem
> (drivers/mtd/mtdchar.c).

No, it's not.  MTD only has three different backing_dev_info instances
which have different flags in the mapping-relevant portion of the
backing_dev. 

>   So at least here you'd need also add a similar exception for
> "mtd_inodefs".

No.  For one thing we don't need any exception for correctnes alone -
even the block device variant would work fine with the default case.
But for mtd specificly we don't need an exception for performance either
given that there are no per-device bdis in mtd.