tmpfs: fix mempolicy object leaks

Fix several mempolicy leaks in the tmpfs mount logic.  These leaks are
slow - on the order of one object leaked per mount attempt.

Leak 1 (umount doesn't free mpol allocated in mount):
    while true; do
        mount -t tmpfs -o mpol=interleave,size=100M nodev /mnt
        umount /mnt
    done

Leak 2 (errors parsing remount options will leak mpol):
    mount -t tmpfs -o size=100M nodev /mnt
    while true; do
        mount -o remount,mpol=interleave,size=x /mnt 2> /dev/null
    done
    umount /mnt

Leak 3 (multiple mpol per mount leak mpol):
    while true; do
        mount -t tmpfs -o mpol=interleave,mpol=interleave,size=100M nodev /mnt
        umount /mnt
    done

This patch fixes all of the above.  I could have broken the patch into
three pieces but is seemed easier to review as one.

[akpm@linux-foundation.org: fix handling of mpol_parse_str() errors, per Hugh]
Signed-off-by: Greg Thelen <gthelen@google.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -2385,6 +2385,7 @@ static int shmem_parse_options(char *options, struct shmem_sb_info *sbinfo,
 			       bool remount)
 {
 	char *this_char, *value, *rest;
+	struct mempolicy *mpol = NULL;
 	uid_t uid;
 	gid_t gid;
 
@@ -2413,7 +2414,7 @@ static int shmem_parse_options(char *options, struct shmem_sb_info *sbinfo,
 			printk(KERN_ERR
 			    "tmpfs: No value for mount option '%s'\n",
 			    this_char);
-			return 1;
+			goto error;
 		}
 
 		if (!strcmp(this_char,"size")) {
@@ -2462,19 +2463,24 @@ static int shmem_parse_options(char *options, struct shmem_sb_info *sbinfo,
 			if (!gid_valid(sbinfo->gid))
 				goto bad_val;
 		} else if (!strcmp(this_char,"mpol")) {
-			if (mpol_parse_str(value, &sbinfo->mpol))
+			mpol_put(mpol);
+			mpol = NULL;
+			if (mpol_parse_str(value, &mpol))
 				goto bad_val;
 		} else {
 			printk(KERN_ERR "tmpfs: Bad mount option %s\n",
 			       this_char);
-			return 1;
+			goto error;
 		}
 	}
+	sbinfo->mpol = mpol;
 	return 0;
 
 bad_val:
 	printk(KERN_ERR "tmpfs: Bad value '%s' for mount option '%s'\n",
 	       value, this_char);
+error:
+	mpol_put(mpol);
 	return 1;
 
 }
@@ -2550,6 +2556,7 @@ static void shmem_put_super(struct super_block *sb)
 	struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
 
 	percpu_counter_destroy(&sbinfo->used_blocks);
+	mpol_put(sbinfo->mpol);
 	kfree(sbinfo);
 	sb->s_fs_info = NULL;
 }