Re: [PATCH 2/7] Assign bdi in super_block

Jan Kara
2009-09-15T10:15:00Z

On Mon 14-09-09 20:36:54, Jens Axboe wrote:
> On Mon, Sep 14 2009, Trond Myklebust wrote:
> > On Mon, 2009-09-14 at 15:02 +0200, Jan Kara wrote:
> > > On Mon 14-09-09 11:36:29, Jens Axboe wrote:
> > > > We do this automatically in get_sb_bdev() from the set_bdev_super()
> > > > callback. Filesystems that have their own private backing_dev_info
> > > > must assign that in ->fill_super().
> > > >
> > > > Note that ->s_bdi assignment is required for proper writeback!
> > > >
> > > > Acked-by: Christoph Hellwig <hch@infradead.org>
> > > > Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
> > > Hmm, looking at this again, I'm not sure this will work for NFS. It seems
> > > to set mapping->backing_dev_info to its private backing dev info for
> > > regular files while it leaves it intact for other inodes (e.g.
> > > directories). I'm not sure why it does so but it seems its inodes end up on
> > > two different BDI lists and thus they wouldn't be synced properly. Trond,
> > > do I read the code properly?
> > > Also we definitely need to set *some* bdi in nfs_get_sb as otherwise sync
> > > won't work for it.
> >
> > There hasn't really been a need for a bdi in NFS other than for the
> > regular file read and writeback code. The main reason for making it
> > private was to ensure that we could set a per-superblock readahead limit
> > that was a decent multiple of the server's preferred read block size.
> >
> > Is there any reason why we couldn't set sb->s_bdi to point to that
> > private bdi?
>
> No, that should work fine. NFS already works fine with the bdi flusher
> threads, so you should just point it at that bdi.
But will it really work well? I mean if we sync the superblock on the
client, it will sync only the private BDI. So it won't sync any directory
inodes because they are on the default_backing_dev_info (NFS leaves
sb->s_bdev at NULL).

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR