mempolicy: fix parsing of tmpfs mpol mount option

Parsing of new mode flags in the tmpfs mpol mount option is slightly broken:

Setting a valid flag works OK:
	#mount -o remount,mpol=bind=static:1-2 /dev/shm
	#mount
	...
	tmpfs on /dev/shm type tmpfs (rw,mpol=bind=static:1-2)
	...

However, we can't remove them or change them, once we've
set a valid flag:

	#mount -o remount,mpol=bind:1-2 /dev/shm
	#mount
	...
	tmpfs on /dev/shm type tmpfs (rw,mpol=bind:1-2)
	...

It SAYS it removed it, but that's just a copy of the input
string.  If we now try to set it to a different flag, we
get:

	#mount -o remount,mpol=bind=relative:1-2 /dev/shm
	mount: /dev/shm not mounted already, or bad option

And on the console, we see:
	tmpfs: Bad value 'bind' for mount option 'mpol'
	                      ^ lost remainder of string

Furthermore, bogus flags are accepted with out error.
Granted, they are a no-op:

	#mount -o remount,mpol=interleave=foo:0-3 /dev/shm
	#mount
	...
	tmpfs on /dev/shm type tmpfs (rw,mpol=interleave=foo:0-3)

Again, that's just a copy of the input string shown by the mount command.

This patch fixes the behavior by pre-zeroing the flags so that only one of the
mutually exclusive flags can be set at one time.  It also reports an error
when an unrecognized flag is specified.

The check for both flags being set is removed because it can't happen with
this implementation.  If we ever want to support multiple non-exclusive flags,
this area will need rework and we will need to check that any mutually
exclusive flags aren't specified.

Signed-off-by: Lee Schermerhorn <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Paul Jackson <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Eric Whitney <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/shmem.c

@@ -1125,20 +1125,26 @@ static int shmem_parse_mpol(char *value, unsigned short *policy,
 			*policy_nodes = node_states[N_HIGH_MEMORY];
 		err = 0;
 	}
+
+	*mode_flags = 0;
 	if (flags) {
+		/*
+		 * Currently, we only support two mutually exclusive
+		 * mode flags.
+		 */
 		if (!strcmp(flags, "static"))
 			*mode_flags |= MPOL_F_STATIC_NODES;
-		if (!strcmp(flags, "relative"))
+		else if (!strcmp(flags, "relative"))
 			*mode_flags |= MPOL_F_RELATIVE_NODES;
-
-		if ((*mode_flags & MPOL_F_STATIC_NODES) &&
-		    (*mode_flags & MPOL_F_RELATIVE_NODES))
-			err = 1;
+		else
+			err = 1;	/* unrecognized flag */
 	}
 out:
 	/* Restore string for error message */
 	if (nodelist)
 		*--nodelist = ':';
+	if (flags)
+		*--flags = '=';
 	return err;
 }