[PATCH v3 1/3] maple_tree: simplify split calculation

[Wei Yang]

The current calculation for splitting nodes tries to enforce a minimum
span on the leaf nodes.  This code is complex and never worked correctly
to begin with, due to the min value being passed as 0 for all leaves.

The calculation should just split the data as equally as possible
between the new nodes.  Note that b_end will be one more than the data,
so the left side is still favoured in the calculation.

The current code may also lead to a deficient node by not leaving enough
data for the right side of the split. This issue is also addressed with
the split calculation change.

[liam: rephrase the change log]

Fixes: 54a611b60590 ("Maple Tree: add new data structure")
Signed-off-by: Wei Yang <richard.weiyang at gmail.com>
CC: Liam R. Howlett <Liam.Howlett at Oracle.com>
CC: Sidhartha Kumar <sidhartha.kumar at oracle.com>
CC: Lorenzo Stoakes <lorenzo.stoakes at oracle.com>
Cc: <stable at vger.kernel.org>

---
v3:
  * Liam helps rephrase the change log
  * add fix tag and cc stable
---
 lib/maple_tree.c | 23 ++++++-----------------
 1 file changed, 6 insertions(+), 17 deletions(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index d0ae808f3a14..4f2950a1c38d 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -1863,11 +1863,11 @@ static inline int mab_no_null_split(struct maple_big_node *b_node,
  * Return: The first split location.  The middle split is set in @mid_split.
  */
 static inline int mab_calc_split(struct ma_state *mas,
-	 struct maple_big_node *bn, unsigned char *mid_split, unsigned long min)
+	 struct maple_big_node *bn, unsigned char *mid_split)
 {
 	unsigned char b_end = bn->b_end;
 	int split = b_end / 2; /* Assume equal split. */
-	unsigned char slot_min, slot_count = mt_slots[bn->type];
+	unsigned char slot_count = mt_slots[bn->type];
 
 	/*
 	 * To support gap tracking, all NULL entries are kept together and a node cannot
@@ -1900,18 +1900,7 @@ static inline int mab_calc_split(struct ma_state *mas,
 		split = b_end / 3;
 		*mid_split = split * 2;
 	} else {
-		slot_min = mt_min_slots[bn->type];
-
 		*mid_split = 0;
-		/*
-		 * Avoid having a range less than the slot count unless it
-		 * causes one node to be deficient.
-		 * NOTE: mt_min_slots is 1 based, b_end and split are zero.
-		 */
-		while ((split < slot_count - 1) &&
-		       ((bn->pivot[split] - min) < slot_count - 1) &&
-		       (b_end - split > slot_min))
-			split++;
 	}
 
 	/* Avoid ending a node on a NULL entry */
@@ -2377,7 +2366,7 @@ static inline struct maple_enode
 static inline unsigned char mas_mab_to_node(struct ma_state *mas,
 	struct maple_big_node *b_node, struct maple_enode **left,
 	struct maple_enode **right, struct maple_enode **middle,
-	unsigned char *mid_split, unsigned long min)
+	unsigned char *mid_split)
 {
 	unsigned char split = 0;
 	unsigned char slot_count = mt_slots[b_node->type];
@@ -2390,7 +2379,7 @@ static inline unsigned char mas_mab_to_node(struct ma_state *mas,
 	if (b_node->b_end < slot_count) {
 		split = b_node->b_end;
 	} else {
-		split = mab_calc_split(mas, b_node, mid_split, min);
+		split = mab_calc_split(mas, b_node, mid_split);
 		*right = mas_new_ma_node(mas, b_node);
 	}
 
@@ -2877,7 +2866,7 @@ static void mas_spanning_rebalance(struct ma_state *mas,
 		mast->bn->b_end--;
 		mast->bn->type = mte_node_type(mast->orig_l->node);
 		split = mas_mab_to_node(mas, mast->bn, &left, &right, &middle,
-					&mid_split, mast->orig_l->min);
+					&mid_split);
 		mast_set_split_parents(mast, left, middle, right, split,
 				       mid_split);
 		mast_cp_to_nodes(mast, left, middle, right, split, mid_split);
@@ -3365,7 +3354,7 @@ static void mas_split(struct ma_state *mas, struct maple_big_node *b_node)
 		if (mas_push_data(mas, height, &mast, false))
 			break;
 
-		split = mab_calc_split(mas, b_node, &mid_split, prev_l_mas.min);
+		split = mab_calc_split(mas, b_node, &mid_split);
 		mast_split_data(&mast, mas, split);
 		/*
 		 * Usually correct, mab_mas_cp in the above call overwrites
-- 
2.34.1