Re: [PATCH 0/6] nilfs2: remove own inode hash table

[Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>]

On Wed, Aug 25, 2010 at 01:20:12AM +0900, Ryusuke Konishi wrote:
> On Tue, 24 Aug 2010 11:32:14 -0400, Christoph Hellwig wrote:
> > On Tue, Aug 24, 2010 at 09:58:23PM +0900, Ryusuke Konishi wrote:
> > > Actually, I recently wrote a patchset to unify all super block
> > > instances on one device into one sb, which makes the inodes outlive
> > > superblock eliminable.
> > 
> > Yes, that's how we handle things everywhere, including btrfs with
> > it's subvolumes and snapshots.
> > 
> > > But the patchset has a problem that it cannot shrink a number of
> > > versions of namespace when unmounting all snapshots and a current time
> > > instance.
> > 
> > Can you explain the issue in more detail?
> 
> Ok, I'll try.
> 
> The current nilfs attaches each snapshot on a mountpoint, instead of
> making a sub directory like ".snap/xxx/" in a namespace.
> 
> In the patchset, I binded
> 
>  a) The root directory of the current filesystem onto sb->s_root and
>     mnt->mnt_root.
> 
>  b) The root directory of each snapshot onto its mnt->mnt_root.
> 
> Then,
> 
> When I unmounted all snapshots and current filesystem,
> nilfs_put_super() was called, but
> 
> generic_shutdown_super() only shrunk the dentry tree of sb->s_root and
> didn't shrink dentry trees of snapshots.
> 
> Thus their inodes and dentries were left in memory despite my
> intention.
> 
> So, my implementation issue here is how to free dentries and inodes of
> snapshots like shrink_dcache_for_umount(sb) does for a single
> namespace.

Use d_obtain_alias() to allocate roots of unattached subtrees.
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