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

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Date:	Fri, 30 Nov 2012 13:58:18 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Patrick McLean <patrick@....mcgill.ca>,
	Patrick McLean <patrickm@...kai.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>
Subject: Re: Regression with initramfs and nfsroot (appears to be in the
 dcache)

On Fri, 2012-11-30 at 02:00 +0000, Al Viro wrote:
> 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?

The reason for the choice of d_drop over d_invalidate() is the d_count
checks. It really doesn't matter whether or not the client thinks it has
users for a directory if the server is telling you that it is ESTALE. So
we force a d_drop to prevent further lookups from finding it.

IOW: It is there in order to fix the case where the user does
'rmdir("foo"); mkdir("foo")' on the server.


-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com