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

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Message-ID: <50B81B11.7050704@gaikai.com>
Date:	Thu, 29 Nov 2012 18:33:53 -0800
From:	Patrick McLean <patrickm@...kai.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Patrick McLean <patrickm@...kai.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	linux-nfs@...r.kernel.org
Subject: Re: Regression with initramfs and nfsroot (appears to be in the dcache)

On 29/11/12 06:00 PM, 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?
> 
> 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);
>  		}
>  	}

Excellent, thanks. Is there any chance this will make it to 3.7? Also we might want to cc stable@ on this as well since it is a regression in 3.6.
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