nfs: use change attribute for NFS re-exports

When exporting NFS, we may as well use the real change attribute
returned by the original server instead of faking up a change attribute
from the ctime.

Note we can't do that by setting I_VERSION--that would also turn on the
logic in iversion.h which treats the lower bit specially, and that
doesn't make sense for NFS.

So instead we define a new export operation for filesystems like NFS
that want to manage the change attribute themselves.

Signed-off-by: J. Bruce Fields <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/nfs/export.c

···
 	return parent;
 }
 
+static u64 nfs_fetch_iversion(struct inode *inode)
+{
+	struct nfs_server *server = NFS_SERVER(inode);
+
+	/* Is this the right call?: */
+	nfs_revalidate_inode(server, inode);
+	/*
+	 * Also, note we're ignoring any returned error.  That seems to be
+	 * the practice for cache consistency information elsewhere in
+	 * the server, but I'm not sure why.
+	 */
+	if (server->nfs_client->rpc_ops->version >= 4)
+		return inode_peek_iversion_raw(inode);
+	else
+		return time_to_chattr(&inode->i_ctime);
+}
+
 const struct export_operations nfs_export_ops = {
 	.encode_fh = nfs_encode_fh,
 	.fh_to_dentry = nfs_fh_to_dentry,
 	.get_parent = nfs_get_parent,
+	.fetch_iversion = nfs_fetch_iversion,
 	.flags = EXPORT_OP_NOWCC|EXPORT_OP_NOSUBTREECHK|
 		EXPORT_OP_CLOSE_BEFORE_UNLINK|EXPORT_OP_REMOTE_FS|
 		EXPORT_OP_NOATOMIC_ATTR,

fs/nfsd/nfsfh.h

···
 #include <linux/sunrpc/svc.h>
 #include <uapi/linux/nfsd/nfsfh.h>
 #include <linux/iversion.h>
+#include <linux/exportfs.h>
 
 static inline __u32 ino_t_to_u32(ino_t ino)
 {
···
 static inline u64 nfsd4_change_attribute(struct kstat *stat,
 					 struct inode *inode)
 {
-	if (IS_I_VERSION(inode)) {
+	if (inode->i_sb->s_export_op->fetch_iversion)
+		return inode->i_sb->s_export_op->fetch_iversion(inode);
+	else if (IS_I_VERSION(inode)) {
 		u64 chattr;
 
 		chattr =  stat->ctime.tv_sec;

include/linux/exportfs.h

···
 			  bool write, u32 *device_generation);
 	int (*commit_blocks)(struct inode *inode, struct iomap *iomaps,
 			     int nr_iomaps, struct iattr *iattr);
+	u64 (*fetch_iversion)(struct inode *);
 #define	EXPORT_OP_NOWCC			(0x1) /* don't collect v3 wcc data */
 #define	EXPORT_OP_NOSUBTREECHK		(0x2) /* no subtree checking */
 #define	EXPORT_OP_CLOSE_BEFORE_UNLINK	(0x4) /* close files before unlink */