Re: Regression with initramfs and nfsroot (appears to be in the dcache)

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

On Thu, Nov 29, 2012 at 05:54:02PM -0800, Patrick McLean wrote:
> > 	Very interesting.  Do you have anything mounted on the corresponding
> > directories on server?  The picture looks like you are getting empty
> > fhandles in readdir+ respons for exactly the same directories that happen
> > to be mountpoints on client.  In any case, we shouldn't do that blind
> > d_drop() - empty fhandles can happen.  The only remaining question is
> > why do they happen on that set of entries.  From my reading of
> > encode_entryplus_baggage() it looks like we have compose_entry_fh()
> > failing for those entries and those entries alone.  One possible cause
> > would be d_mountpoint(dchild) being true on server.  If it is true, we
> > can declare the case closed; if not, I really wonder what's going on.
> 
> Those directories do have the server's own copies of the said directories bind mounted at the moment in a separate mount namespace.
> 
> Unmounting those directories on the server does appear to stop the WARN_ON from triggering.

OK, that settles it.  WARN_ON() and printks in the area can be dropped;
the right fix is below.  However, there's a similar place in cifs that
also needs to be dealt with and I really, really wonder why the hell do
we do d_drop() in nfs_revalidate_lookup().  It's not relevant in this
bug, but I would like to understand what's wrong with simply returning
0 from ->d_revalidate() and letting the caller (in fs/namei.c) take care
of unhashing, etc. itself.  Would make have_submounts() in there pointless
as well - we could just return 0 and let d_invalidate() take care of the
checks...  Trond?

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -450,7 +450,8 @@ void nfs_prime_dcache(struct dentry *parent, struct nfs_entry *entry)
 			nfs_refresh_inode(dentry->d_inode, entry->fattr);
 			goto out;
 		} else {
-			d_drop(dentry);
+			if (d_invalidate(dentry) != 0)
+				goto out;
 			dput(dentry);
 		}
 	}