[PATCH] relative atime

Add "relatime" (relative atime) support. Relative atime only updates the
atime if the previous atime is older than the mtime or ctime. Like
noatime, but useful for applications like mutt that need to know when a
file has been read since it was last modified.

A corresponding patch against mount(8) is available at
http://userweb.kernel.org/~akpm/mount-relative-atime.txt

Signed-off-by: Valerie Henson <[e-mail]>
Cc: Mark Fasheh <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Karel Zak <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/inode.c

···
 			return;
 		if ((mnt->mnt_flags & MNT_NODIRATIME) && S_ISDIR(inode->i_mode))
 			return;
+
+		if (mnt->mnt_flags & MNT_RELATIME) {
+			/*
+			 * With relative atime, only update atime if the
+			 * previous atime is earlier than either the ctime or
+			 * mtime.
+			 */
+			if (timespec_compare(&inode->i_mtime,
+						&inode->i_atime) < 0 &&
+			    timespec_compare(&inode->i_ctime,
+						&inode->i_atime) < 0)
+				return;
+		}
 	}
 
 	now = current_fs_time(inode->i_sb);
-	if (!timespec_equal(&inode->i_atime, &now)) {
-		inode->i_atime = now;
-		mark_inode_dirty_sync(inode);
-	}
+	if (timespec_equal(&inode->i_atime, &now))
+		return;
+
+	inode->i_atime = now;
+	mark_inode_dirty_sync(inode);
 }
 EXPORT_SYMBOL(touch_atime);
 

fs/namespace.c

···
 		{ MNT_NOEXEC, ",noexec" },
 		{ MNT_NOATIME, ",noatime" },
 		{ MNT_NODIRATIME, ",nodiratime" },
+		{ MNT_RELATIME, ",relatime" },
 		{ 0, NULL }
 	};
 	struct proc_fs_info *fs_infop;
···
 		mnt_flags |= MNT_NOATIME;
 	if (flags & MS_NODIRATIME)
 		mnt_flags |= MNT_NODIRATIME;
+	if (flags & MS_RELATIME)
+		mnt_flags |= MNT_RELATIME;
 
 	flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
-		   MS_NOATIME | MS_NODIRATIME);
+		   MS_NOATIME | MS_NODIRATIME | MS_RELATIME);
 
 	/* ... and get the mountpoint */
 	retval = path_lookup(dir_name, LOOKUP_FOLLOW, &nd);

include/linux/fs.h

···
 #define MS_PRIVATE	(1<<18)	/* change to private */
 #define MS_SLAVE	(1<<19)	/* change to slave */
 #define MS_SHARED	(1<<20)	/* change to shared */
+#define MS_RELATIME	(1<<21)	/* Update atime relative to mtime/ctime. */
 #define MS_ACTIVE	(1<<30)
 #define MS_NOUSER	(1<<31)
 

include/linux/mount.h

···
 #define MNT_NOEXEC	0x04
 #define MNT_NOATIME	0x08
 #define MNT_NODIRATIME	0x10
+#define MNT_RELATIME	0x20
 
 #define MNT_SHRINKABLE	0x100