GFS2: Fix non-recursive truncate bug

Before this patch if you truncated a file to a smaller size it
wasn't freeing all the blocks properly. There are two reasons.

First, the metapath comparison was not comparing previous heights.
I added a function, mp_eq_to_hgt, which checks the metapath at
all heights prior to the target height.

Second, in function find_nonnull_ptr, it needed to zero out all
pointers for heights following the target height. Translated into
decimal integer terms, this way a number like 299, when incremented,
becomes 300, not 399. The 2 gets incremented to 3, and the following
digits need to be reset.

These two things allow the truncate state machine to properly find
the blocks it needs to delete.
Signed-off-by: NBob Peterson <rpeterso@redhat.com>

fs/gfs2/bmap.c

@@ -1104,8 +1104,15 @@ static bool find_nonnull_ptr(struct gfs2_sbd *sdp, struct metapath *mp,
 
 	while (true) {
 		ptr = metapointer(h, mp);
-		if (*ptr) /* if we have a non-null pointer */
+		if (*ptr) { /* if we have a non-null pointer */
+			/* Now zero the metapath after the current height. */
+			h++;
+			if (h < GFS2_MAX_META_HEIGHT)
+				memset(&mp->mp_list[h], 0,
+				       (GFS2_MAX_META_HEIGHT - h) *
+				       sizeof(mp->mp_list[0]));
 			return true;
+		}
 
 		if (mp->mp_list[h] < ptrs)
 			mp->mp_list[h]++;
@@ -1121,6 +1128,13 @@ enum dealloc_states {
 	DEALLOC_DONE = 3,       /* process complete */
 };
 
+static bool mp_eq_to_hgt(struct metapath *mp, __u16 *nbof, unsigned int h)
+{
+	if (memcmp(mp->mp_list, nbof, h * sizeof(mp->mp_list[0])))
+		return false;
+	return true;
+}
+
 /**
  * trunc_dealloc - truncate a file down to a desired size
  * @ip: inode to truncate
@@ -1198,8 +1212,7 @@ static int trunc_dealloc(struct gfs2_inode *ip, u64 newsize)
 			/* If we're truncating to a non-zero size and the mp is
 			   at the beginning of file for the strip height, we
 			   need to preserve the first metadata pointer. */
-			preserve1 = (newsize &&
-				     (mp.mp_list[mp_h] == nbof[mp_h]));
+			preserve1 = (newsize && mp_eq_to_hgt(&mp, nbof, mp_h));
 			bh = mp.mp_bh[mp_h];
 			gfs2_assert_withdraw(sdp, bh);
 			if (gfs2_assert_withdraw(sdp,