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    SubjectRe: [PATCH 2/7] Assign bdi in super_block
    On Tue, Sep 15 2009, Jan Kara wrote:
    > 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).

    No you are right, it wont be complete. I talked to Chris about this
    yesterday, and we agree that the best option is to give NFS the 'btrfs
    treatment' bdi wise.

    --
    Jens Axboe



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