xfs: refactor delalloc indlen reservation split into helper

The delayed allocation indirect reservation splitting code is not
sufficient in some cases where a delalloc extent is split in two. In
preparation for enhancements to this code, refactor the current indlen
distribution algorithm into a new helper function.

[dchinner: rename temp, temp2 variables]

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/libxfs/xfs_bmap.c

@@ -4720,6 +4720,47 @@ error0:
 	return error;
 }
 
+/*
+ * When a delalloc extent is split (e.g., due to a hole punch), the original
+ * indlen reservation must be shared across the two new extents that are left
+ * behind.
+ *
+ * Given the original reservation and the worst case indlen for the two new
+ * extents (as calculated by xfs_bmap_worst_indlen()), split the original
+ * reservation fairly across the two new extents.
+ */
+static void
+xfs_bmap_split_indlen(
+	xfs_filblks_t			ores,		/* original res. */
+	xfs_filblks_t			*indlen1,	/* ext1 worst indlen */
+	xfs_filblks_t			*indlen2)	/* ext2 worst indlen */
+{
+	xfs_filblks_t			len1 = *indlen1;
+	xfs_filblks_t			len2 = *indlen2;
+	xfs_filblks_t			nres = len1 + len2; /* new total res. */
+
+	/*
+	 * The only blocks available are those reserved for the original extent.
+	 * Therefore, we have to skim blocks off each of the new reservations so
+	 * long as the new total reservation is greater than the original.
+	 */
+	while (nres > ores) {
+		if (len1) {
+			len1--;
+			nres--;
+		}
+		if (nres == ores)
+			break;
+		if (len2) {
+			len2--;
+			nres--;
+		}
+	}
+
+	*indlen1 = len1;
+	*indlen2 = len2;
+}
+
 /*
  * Called by xfs_bmapi to update file extent records and the btree
  * after removing space (or undoing a delayed allocation).
@@ -4985,27 +5026,21 @@ xfs_bmap_del_extent(
 				XFS_IFORK_NEXTENTS(ip, whichfork) + 1);
 		} else {
 			ASSERT(whichfork == XFS_DATA_FORK);
-			temp = xfs_bmap_worst_indlen(ip, temp);
-			xfs_bmbt_set_startblock(ep, nullstartblock((int)temp));
-			temp2 = xfs_bmap_worst_indlen(ip, temp2);
-			new.br_startblock = nullstartblock((int)temp2);
+
+			/*
+			 * Distribute the original indlen reservation across the
+			 * two new extents.
+			 */
+			temp = xfs_bmap_worst_indlen(ip, got.br_blockcount);
+			temp2 = xfs_bmap_worst_indlen(ip, new.br_blockcount);
+			xfs_bmap_split_indlen(da_old, &temp, &temp2);
 			da_new = temp + temp2;
-			while (da_new > da_old) {
-				if (temp) {
-					temp--;
-					da_new--;
-					xfs_bmbt_set_startblock(ep,
-						nullstartblock((int)temp));
-				}
-				if (da_new == da_old)
-					break;
-				if (temp2) {
-					temp2--;
-					da_new--;
-					new.br_startblock =
-						nullstartblock((int)temp2);
-				}
-			}
+
+			/*
+			 * Set the reservation for each extent.
+			 */
+			xfs_bmbt_set_startblock(ep, nullstartblock((int)temp));
+			new.br_startblock = nullstartblock((int)temp2);
 		}
 		trace_xfs_bmap_post_update(ip, *idx, state, _THIS_IP_);
 		xfs_iext_insert(ip, *idx + 1, 1, &new, state);