Commit 4f6a6bed authored by Wei Yang's avatar Wei Yang Committed by Andrew Morton
Browse files

maple_tree: simplify split calculation 

Patch series "simplify split calculation", v3.


This patch (of 3):

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.Howlett@Oracle.com: rephrase the change log]
Link: https://lkml.kernel.org/r/20241113031616.10530-1-richard.weiyang@gmail.com
Link: https://lkml.kernel.org/r/20241113031616.10530-2-richard.weiyang@gmail.com


Fixes: 54a611b6 ("Maple Tree: add new data structure")
Signed-off-by: default avatarWei Yang <richard.weiyang@gmail.com>
Reviewed-by: default avatarLiam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 146ca401

lib/maple_tree.c

+6 −17
Original line number Diff line number Diff line
@@ -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