UBIFS: fix available blocks count

Take into account that 2 eraseblocks are never available because
they are reserved for the index. This gives more realistic count
of FS blocks.

To avoid future confusions like this, introduce a constant.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

fs/ubifs/budget.c

@@ -280,13 +280,8 @@ int ubifs_calc_min_idx_lebs(struct ubifs_info *c)
 	 * extra LEB to compensate.
 	 */
 	ret += 1;
-	/*
-	 * At present the index needs at least 2 LEBs: one for the index head
-	 * and one for in-the-gaps method (which currently does not cater for
-	 * the index head and so excludes it from consideration).
-	 */
-	if (ret < 2)
-		ret = 2;
+	if (ret < MIN_INDEX_LEBS)
+		ret = MIN_INDEX_LEBS;
 	return ret;
 }
 

fs/ubifs/super.c

@@ -695,9 +695,10 @@ static int init_constants_late(struct ubifs_info *c)
 	 * necessary to report something for the 'statfs()' call.
 	 *
 	 * Subtract the LEB reserved for GC, the LEB which is reserved for
-	 * deletions, and assume only one journal head is available.
+	 * deletions, minimum LEBs for the index, and assume only one journal
+	 * head is available.
 	 */
-	tmp64 = c->main_lebs - 2 - c->jhead_cnt + 1;
+	tmp64 = c->main_lebs - 1 - 1 - MIN_INDEX_LEBS - c->jhead_cnt + 1;
 	tmp64 *= (uint64_t)c->leb_size - c->leb_overhead;
 	tmp64 = ubifs_reported_space(c, tmp64);
 	c->block_cnt = tmp64 >> UBIFS_BLOCK_SHIFT;

fs/ubifs/ubifs.h

@@ -63,6 +63,14 @@
 #define SQNUM_WARN_WATERMARK 0xFFFFFFFF00000000ULL
 #define SQNUM_WATERMARK      0xFFFFFFFFFF000000ULL
 
+/*
+ * Minimum amount of LEBs reserved for the index. At present the index needs at
+ * least 2 LEBs: one for the index head and one for in-the-gaps method (which
+ * currently does not cater for the index head and so excludes it from
+ * consideration).
+ */
+#define MIN_INDEX_LEBS 2
+
 /* Minimum amount of data UBIFS writes to the flash */
 #define MIN_WRITE_SZ (UBIFS_DATA_NODE_SZ + 8)