maple_tree: add sufficient height

In order to support rebalancing and spanning stores using less than the
worst case number of nodes, we need to track more than just the vacant
height.  Using only vacant height to reduce the worst case maple node
allocation count can lead to a shortcoming of nodes in the following
scenarios.

For rebalancing writes, when a leaf node becomes insufficient, it may be
combined with a sibling into a single node.  This means that the parent
node which has entries for this children will lose one entry.  If this
parent node was just meeting the minimum entries, losing one entry will
now cause this parent node to be insufficient.  This leads to a cascading
operation of rebalancing at different levels and can lead to more node
allocations than simply using vacant height can return.

For spanning writes, a similar situation occurs.  At the location at which
a spanning write is detected, the number of ancestor nodes may similarly
need to rebalanced into a smaller number of nodes and the same cascading
situation could occur.

To use less than the full height of the tree for the number of
allocations, we also need to track the height at which a non-leaf node
cannot become insufficient.  This means even if a rebalance occurs to a
child of this node, it currently has enough entries that it can lose one
without any further action.  This field is stored in the maple write state
as sufficient height.  In mas_prealloc_calc() when figuring out how many
nodes to allocate, we check if the vacant node is lower in the tree than a
sufficient node (has a larger value).  If it is, we cannot use the vacant
height and must use the difference in the height and sufficient height as
the basis for the number of nodes needed.

An off by one bug was also discovered in mast_overflow() where it is using
>= rather than >.  This caused extra iterations of the
mas_spanning_rebalance() loop and lead to unneeded allocations.  A test is
also added to check the number of allocations is correct.

Link: https://lkml.kernel.org/r/20250410191446.2474640-6-sidhartha.kumar@oracle.com
Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 16 insertions, 3 deletions

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 5610b3742a79..aa139668bcae 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -2741,7 +2741,7 @@ static inline bool mast_sufficient(struct maple_subtree_state *mast)
  */
 static inline bool mast_overflow(struct maple_subtree_state *mast)
 {
-	if (mast->bn->b_end >= mt_slot_count(mast->orig_l->node))
+	if (mast->bn->b_end > mt_slot_count(mast->orig_l->node))
 		return true;
 
 	return false;
@@ -3550,6 +3550,13 @@ static bool mas_wr_walk(struct ma_wr_state *wr_mas)
 		if (mas->end < mt_slots[wr_mas->type] - 1)
 			wr_mas->vacant_height = mas->depth + 1;
 
+		if (ma_is_root(mas_mn(mas))) {
+			/* root needs more than 2 entries to be sufficient + 1 */
+			if (mas->end > 2)
+				wr_mas->sufficient_height = 1;
+		} else if (mas->end > mt_min_slots[wr_mas->type] + 1)
+			wr_mas->sufficient_height = mas->depth + 1;
+
 		mas_wr_walk_traverse(wr_mas);
 	}
 
@@ -4185,13 +4192,19 @@ static inline int mas_prealloc_calc(struct ma_wr_state *wr_mas, void *entry)
 			ret = 0;
 		break;
 	case wr_spanning_store:
-		WARN_ON_ONCE(ret != height * 3 + 1);
+		if (wr_mas->sufficient_height < wr_mas->vacant_height)
+			ret = (height - wr_mas->sufficient_height) * 3 + 1;
+		else
+			ret = delta * 3 + 1;
 		break;
 	case wr_split_store:
 		ret = delta * 2 + 1;
 		break;
 	case wr_rebalance:
-		ret = height * 2 + 1;
+		if (wr_mas->sufficient_height < wr_mas->vacant_height)
+			ret = (height - wr_mas->sufficient_height) * 2 + 1;
+		else
+			ret = delta * 2 + 1;
 		break;
 	case wr_node_store:
 		ret = mt_in_rcu(mas->tree) ? 1 : 0;