Re: vfs-scale, d_revalidate from nfsd

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Jan 14, 2011 at 01:57:55PM +1100, Nick Piggin wrote:
> On Fri, Jan 14, 2011 at 1:03 AM, J. R. Okajima <hooanon05@yahoo.co.jp> wrote:
> >
> > NFSD calls filesystem's ->d_revalidate() with the parameter nd == NULL.
> > So every
> > ? ? ? ?if (nd->flags & LOOKUP_RCU)
> > ? ? ? ? ? ? ? ?return -ECHILD;
> > code which was added to ->d_revalidate() of FS which supports NFS
> > exporting will crash.
> >
> > If we rewrite it as
> > ? ? ? ?if (nd && (nd->flags & LOOKUP_RCU))
> > ? ? ? ? ? ? ? ?return -ECHILD;
> > the problem may not occur.
> > But I am not sure whether lookup_one_len() call in NFSD support rcu-walk.
> 
> Ah, good catch.
> 
> I'm going to change the d_revalidate API so it takes and inode and rcu-walk
> flag parameter to make it easier for filesystems to implement rcu-walk.
> 
> That will take care of this NULL nd case.

Oh, for fuck sake...  Would you at least mind posting your API change
description to fsdevel before going for it?

->d_revalidate() is one sick puppy and it's intimately involved in atomic
open rewrite.  Please, *please* don't make that shit even messier...