Re: [PATCH] fs: pipe/sockets/anon dentries should not have a parent

[Christoph Hellwig <hch@infradead.org>]

On Fri, Nov 21, 2008 at 04:13:38PM +0100, Eric Dumazet wrote:
> [PATCH] fs: pipe/sockets/anon dentries should not have a parent
>
> Linking pipe/sockets/anon dentries to one root 'parent' has no functional
> impact at all, but a scalability one.
>
> We can avoid touching a cache line at allocation stage (inside d_alloc(), no need
> to touch root->d_count), but also at freeing time (in d_kill, decrementing d_count)
> We avoid an expensive atomic_dec_and_lock() call on the root dentry.
>
> If we correct dnotify_parent() and inotify_d_instantiate() to take into account
> a NULL d_parent, we can call d_alloc() with a NULL parent instead of root dentry.

Sorry folks, but a NULL d_parent is a no-go from the VFS perspective,
but you can set d_parent to the dentry itself which is the magic used
for root of tree dentries.  They should also be marked
DCACHE_DISCONNECTED to make sure this is not unexpected.

And this kind of stuff really needs to go through -fsdevel.