Re: [PATCH 1/3] NFS: Don't invalidate a submounted dentry in nfs_prime_dcache()

[NeilBrown <neilb@suse.de>]

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On Mon, 23 Feb 2015 19:14:55 -0500 Trond Myklebust
<trond.myklebust@primarydata.com> wrote:

> If we're traversing a directory which contains a submounted filesystem,
> or one that has a referral, the NFS server that is processing the READDIR
> request will often return information for the underlying (mounted-on)
> directory. It may, or may not, also return filehandle information.
> 
> If this happens, and the lookup in nfs_prime_dcache() returns the
> dentry for the submounted directory, the filehandle comparison will
> fail, and we call d_invalidate(). Post-commit 8ed936b5671bf
> ("vfs: Lazily remove mounts on unlinked files and directories."), this
> means the entire subtree is unmounted.
> 
> The following minimal patch addresses this problem by punting on
> the invalidation if there is a submount.
> 
> Kudos to Neil Brown <neilb@suse.de> for having tracked down this
> issue (see link).
> 
> Reported-by: Nix <nix@esperi.org.uk>
> Link: http://lkml.kernel.org/r/87iofju9ht.fsf@spindle.srvr.nix
> Cc: stable@vger.kernel.org # 3.18+
> Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
> ---
>  fs/nfs/dir.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> index 9b0c55cb2a2e..0da617a61c0b 100644
> --- a/fs/nfs/dir.c
> +++ b/fs/nfs/dir.c
> @@ -479,6 +479,10 @@ void nfs_prime_dcache(struct dentry *parent, struct nfs_entry *entry)
>  
>  	dentry = d_lookup(parent, &filename);
>  	if (dentry != NULL) {
> +		/* Is there a mountpoint here? If so, just exit */
> +		if (!nfs_fsid_equal(&NFS_SB(dentry->d_sb)->fsid,
> +					&entry->fattr->fsid))
> +			goto out;

Surely we should  only consider fattr->fsid if NFS_ATTR_FATTR_FSID is set.
In the case of the Linux NFSv3 server, if this were a mountpoint on the
server, then NFS_ATTR_FATTR_FSID will not be set (as the server returns
neither postop attrs nor filehandle).

I realise that this issue is addressed in the subsequent patch (3/3), but
that isn't tagged for -stable, and this is.

NeilBrown



>  		if (nfs_same_file(dentry, entry)) {
>  			nfs_set_verifier(dentry, nfs_save_change_attribute(dir));
>  			status = nfs_refresh_inode(dentry->d_inode, entry->fattr);


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