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maple_tree: simplify split calculation

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commit 4f6a6bed0bfef4b966f076f33eb4f5547226056a upstream.

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: 54a611b605 ("Maple Tree: add new data structure")
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Liam 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Wei Yang 2024-11-13 03:16:14 +00:00 committed by Greg Kroah-Hartman
parent c4720d0490
commit 09b288fb7f
1 changed files with 6 additions and 17 deletions
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23
lib/maple_tree.c
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@ -1890,11 +1890,11 @@ 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
@ -1927,18 +1927,7 @@ 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 */
@ -2664,7 +2653,7 @@ 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];
@ -2677,7 +2666,7 @@ 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);
}
@ -3076,7 +3065,7 @@ static int 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);
@ -3591,7 +3580,7 @@ static int 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