vfs: update ovl inode before relatime check

On overlayfs relatime_need_update() needs inode times to be correct on
overlay inode.  But i_mtime and i_ctime are updated by filesystem code on
underlying inode only, so they will be out-of-date on the overlay inode.

This patch copies the times from the underlying inode if needed.  This
can't be done if called from RCU lookup (link following) but link m/ctime
are not updated by fs, so this is all right.

This patch doesn't change functionality for anything but overlayfs.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/inode.c

@@ -1535,17 +1535,37 @@ sector_t bmap(struct inode *inode, sector_t block)
 }
 EXPORT_SYMBOL(bmap);
 
+/*
+ * Update times in overlayed inode from underlying real inode
+ */
+static void update_ovl_inode_times(struct dentry *dentry, struct inode *inode,
+			       bool rcu)
+{
+	if (!rcu) {
+		struct inode *realinode = d_real_inode(dentry);
+
+		if (unlikely(inode != realinode) &&
+		    (!timespec_equal(&inode->i_mtime, &realinode->i_mtime) ||
+		     !timespec_equal(&inode->i_ctime, &realinode->i_ctime))) {
+			inode->i_mtime = realinode->i_mtime;
+			inode->i_ctime = realinode->i_ctime;
+		}
+	}
+}
+
 /*
  * With relative atime, only update atime if the previous atime is
  * earlier than either the ctime or mtime or if at least a day has
  * passed since the last atime update.
  */
-static int relatime_need_update(struct vfsmount *mnt, struct inode *inode,
-			     struct timespec now)
+static int relatime_need_update(const struct path *path, struct inode *inode,
+				struct timespec now, bool rcu)
 {
 
-	if (!(mnt->mnt_flags & MNT_RELATIME))
+	if (!(path->mnt->mnt_flags & MNT_RELATIME))
 		return 1;
+
+	update_ovl_inode_times(path->dentry, inode, rcu);
 	/*
 	 * Is mtime younger than atime? If yes, update atime:
 	 */
@@ -1612,7 +1632,8 @@ static int update_time(struct inode *inode, struct timespec *time, int flags)
  *	This function automatically handles read only file systems and media,
  *	as well as the "noatime" flag and inode specific "noatime" markers.
  */
-bool atime_needs_update(const struct path *path, struct inode *inode)
+bool __atime_needs_update(const struct path *path, struct inode *inode,
+			  bool rcu)
 {
 	struct vfsmount *mnt = path->mnt;
 	struct timespec now;
@@ -1638,7 +1659,7 @@ bool atime_needs_update(const struct path *path, struct inode *inode)
 
 	now = current_fs_time(inode->i_sb);
 
-	if (!relatime_need_update(mnt, inode, now))
+	if (!relatime_need_update(path, inode, now, rcu))
 		return false;
 
 	if (timespec_equal(&inode->i_atime, &now))
@@ -1653,7 +1674,7 @@ void touch_atime(const struct path *path)
 	struct inode *inode = d_inode(path->dentry);
 	struct timespec now;
 
-	if (!atime_needs_update(path, inode))
+	if (!__atime_needs_update(path, inode, false))
 		return;
 
 	if (!sb_start_write_trylock(inode->i_sb))

fs/internal.h

@@ -120,6 +120,15 @@ extern long prune_icache_sb(struct super_block *sb, struct shrink_control *sc);
 extern void inode_add_lru(struct inode *inode);
 extern int dentry_needs_remove_privs(struct dentry *dentry);
 
+extern bool __atime_needs_update(const struct path *, struct inode *, bool);
+static inline bool atime_needs_update_rcu(const struct path *path,
+					  struct inode *inode)
+{
+	return __atime_needs_update(path, inode, true);
+}
+
+extern bool atime_needs_update_rcu(const struct path *, struct inode *);
+
 /*
  * fs-writeback.c
  */

fs/namei.c

@@ -1015,7 +1015,7 @@ const char *get_link(struct nameidata *nd)
 	if (!(nd->flags & LOOKUP_RCU)) {
 		touch_atime(&last->link);
 		cond_resched();
-	} else if (atime_needs_update(&last->link, inode)) {
+	} else if (atime_needs_update_rcu(&last->link, inode)) {
 		if (unlikely(unlazy_walk(nd, NULL, 0)))
 			return ERR_PTR(-ECHILD);
 		touch_atime(&last->link);

include/linux/fs.h

@@ -2007,7 +2007,6 @@ enum file_time_flags {
 	S_VERSION = 8,
 };
 
-extern bool atime_needs_update(const struct path *, struct inode *);
 extern void touch_atime(const struct path *);
 static inline void file_accessed(struct file *file)
 {