ext4: clean up mballoc criteria comments

Line wrap and slightly clarify the comments describing mballoc's
cirtiera.

Define EXT4_MB_NUM_CRS as part of the enum, so that it will
automatically get updated when criteria is added or removed.

Also fix a potential unitialized use of 'cr' variable if
CONFIG_EXT4_DEBUG is enabled.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: tytso

fs/ext4/ext4.h

@@ -128,47 +128,52 @@ enum SHIFT_DIRECTION {
 };
 
 /*
- * Number of criterias defined. For each criteria, mballoc has slightly
- * different way of finding the required blocks nad usually, higher the
- * criteria the slower the allocation. We start at lower criterias and keep
- * falling back to higher ones if we are not able to find any blocks.
- */
-#define EXT4_MB_NUM_CRS 5
-/*
- * All possible allocation criterias for mballoc. Lower are faster.
+ * For each criteria, mballoc has slightly different way of finding
+ * the required blocks nad usually, higher the criteria the slower the
+ * allocation.  We start at lower criterias and keep falling back to
+ * higher ones if we are not able to find any blocks.  Lower (earlier)
+ * criteria are faster.
  */
 enum criteria {
 	/*
-	 * Used when number of blocks needed is a power of 2. This doesn't
-	 * trigger any disk IO except prefetch and is the fastest criteria.
+	 * Used when number of blocks needed is a power of 2. This
+	 * doesn't trigger any disk IO except prefetch and is the
+	 * fastest criteria.
 	 */
 	CR_POWER2_ALIGNED,
 
 	/*
-	 * Tries to lookup in-memory data structures to find the most suitable
-	 * group that satisfies goal request. No disk IO except block prefetch.
+	 * Tries to lookup in-memory data structures to find the most
+	 * suitable group that satisfies goal request. No disk IO
+	 * except block prefetch.
 	 */
 	CR_GOAL_LEN_FAST,
 
         /*
-	 * Same as CR_GOAL_LEN_FAST but is allowed to reduce the goal length to
-         * the best available length for faster allocation.
+	 * Same as CR_GOAL_LEN_FAST but is allowed to reduce the goal
+         * length to the best available length for faster allocation.
 	 */
 	CR_BEST_AVAIL_LEN,
 
 	/*
-	 * Reads each block group sequentially, performing disk IO if necessary, to
-	 * find find_suitable block group. Tries to allocate goal length but might trim
-	 * the request if nothing is found after enough tries.
+	 * Reads each block group sequentially, performing disk IO if
+	 * necessary, to find find_suitable block group. Tries to
+	 * allocate goal length but might trim the request if nothing
+	 * is found after enough tries.
 	 */
 	CR_GOAL_LEN_SLOW,
 
 	/*
-	 * Finds the first free set of blocks and allocates those. This is only
-	 * used in rare cases when CR_GOAL_LEN_SLOW also fails to allocate
-	 * anything.
+	 * Finds the first free set of blocks and allocates
+	 * those. This is only used in rare cases when
+	 * CR_GOAL_LEN_SLOW also fails to allocate anything.
 	 */
 	CR_ANY_FREE,
+
+	/*
+	 * Number of criterias defined.
+	 */
+	EXT4_MB_NUM_CRS
 };
 
 /* criteria below which we use fast block scanning and avoid unnecessary IO */

fs/ext4/mballoc.c

@@ -1035,11 +1035,9 @@ static void ext4_mb_choose_next_group_best_avail(struct ext4_allocation_context
 
 		if (num_stripe_clusters > 0) {
 			/*
-			 * Try to round up the adjusted goal to stripe size
-			 * (in cluster units) multiple for efficiency.
-			 *
-			 * XXX: Is s->stripe always a power of 2? In that case
-			 * we can use the faster round_up() variant.
+			 * Try to round up the adjusted goal length to
+			 * stripe size (in cluster units) multiple for
+			 * efficiency.
 			 */
 			ac->ac_g_ex.fe_len = roundup(ac->ac_g_ex.fe_len,
 						     num_stripe_clusters);
@@ -2758,7 +2756,7 @@ static noinline_for_stack int
 ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 {
 	ext4_group_t prefetch_grp = 0, ngroups, group, i;
-	enum criteria cr, new_cr;
+	enum criteria new_cr, cr = CR_GOAL_LEN_FAST;
 	int err = 0, first_err = 0;
 	unsigned int nr = 0, prefetch_ios = 0;
 	struct ext4_sb_info *sbi;
@@ -2815,12 +2813,13 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 		spin_unlock(&sbi->s_md_lock);
 	}
 
-	/* Let's just scan groups to find more-less suitable blocks */
-	cr = ac->ac_2order ? CR_POWER2_ALIGNED : CR_GOAL_LEN_FAST;
 	/*
-	 * cr == CR_POWER2_ALIGNED try to get exact allocation,
-	 * cr == CR_ANY_FREE try to get anything
+	 * Let's just scan groups to find more-less suitable blocks We
+	 * start with CR_GOAL_LEN_FAST, unless it is power of 2
+	 * aligned, in which case let's do that faster approach first.
 	 */
+	if (ac->ac_2order)
+		cr = CR_POWER2_ALIGNED;
 repeat:
 	for (; cr < EXT4_MB_NUM_CRS && ac->ac_status == AC_STATUS_CONTINUE; cr++) {
 		ac->ac_criteria = cr;