[PATCH 5/5] maple_tree: add sufficient height

[Sidhartha Kumar]

If a parent node is vacant but holds mt_min_slots + 1 entries,
rebalancing with a leaf node could cause this parent node to become
insufficient. This will lead to another level of rebalancing in the tree
and requires more node allocations. Therefore, we also have to track the
level at which there is a node with > mt_min_slots entries. We can use
this as the worst case for the wr_rebalance case.

Signed-off-by: Sidhartha Kumar <sidhartha.kumar at oracle.com>
---
 include/linux/maple_tree.h       |  4 +++-
 lib/maple_tree.c                 | 14 +++++++++++++-
 tools/testing/radix-tree/maple.c | 28 ++++++++++++++++++++++++++++
 3 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/include/linux/maple_tree.h b/include/linux/maple_tree.h
index 7d777aa2d9ed..37dc9525dff6 100644
--- a/include/linux/maple_tree.h
+++ b/include/linux/maple_tree.h
@@ -464,6 +464,7 @@ struct ma_wr_state {
 	void *entry;			/* The entry to write */
 	void *content;			/* The existing entry that is being overwritten */
 	unsigned char vacant_height;	/* Depth of lowest node with free space */
+	unsigned char sufficient_height;/* Depth of lowest node with min sufficiency + 1 nodes */
 };
 
 #define mas_lock(mas)           spin_lock(&((mas)->tree->ma_lock))
@@ -499,7 +500,8 @@ struct ma_wr_state {
 		.mas = ma_state,					\
 		.content = NULL,					\
 		.entry = wr_entry,					\
-		.vacant_height = 0					\
+		.vacant_height = 0,					\
+		.sufficient_height = 0					\
 	}
 
 #define MA_TOPIARY(name, tree)						\
diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 59c5c3f8db30..3d39773601d3 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -3558,6 +3558,14 @@ 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;
+
+		if (mas->end > mt_min_slots[wr_mas->type] + 1)
+			wr_mas->sufficient_height = mas->depth + 1;
+
 		mas_wr_walk_traverse(wr_mas);
 	}
 
@@ -4198,7 +4206,11 @@ static inline int mas_prealloc_calc(struct ma_wr_state *wr_mas, void *entry)
 		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;
+			break;
+		}
+		ret = delta * 2 + 1;
 		break;
 	case wr_node_store:
 		ret = mt_in_rcu(mas->tree) ? 1 : 0;
diff --git a/tools/testing/radix-tree/maple.c b/tools/testing/radix-tree/maple.c
index bc8b107e0177..f55ab8c8b491 100644
--- a/tools/testing/radix-tree/maple.c
+++ b/tools/testing/radix-tree/maple.c
@@ -36329,6 +36329,30 @@ static noinline void __init check_mtree_dup(struct maple_tree *mt)
 
 extern void test_kmem_cache_bulk(void);
 
+/*
+ * Test to check the path of a spanning rebalance which results in
+ * a collapse where the rebalancing of the child node leads to
+ * insufficieny in the parent node.
+ */
+static void check_collapsing_rebalance(struct maple_tree *mt)
+{
+	int i = 0;
+	MA_STATE(mas, mt, ULONG_MAX, ULONG_MAX);
+
+	/* create a height 4 tree */
+	while (mt_height(mt) < 4) {
+		mtree_store_range(mt, i, i + 10, xa_mk_value(i), GFP_KERNEL);
+		i += 9;
+	}
+
+	/* delete all entries one at a time, starting from the right */
+	do {
+		mas_erase(&mas);
+	} while (mas_prev(&mas, 0) != NULL);
+
+	mtree_unlock(mt);
+}
+
 /* callback function used for check_nomem_writer_race() */
 static void writer2(void *maple_tree)
 {
@@ -36495,6 +36519,10 @@ void farmer_tests(void)
 	check_spanning_write(&tree);
 	mtree_destroy(&tree);
 
+	mt_init_flags(&tree, MT_FLAGS_ALLOC_RANGE);
+	check_collapsing_rebalance(&tree);
+	mtree_destroy(&tree);
+
 	mt_init_flags(&tree, MT_FLAGS_ALLOC_RANGE);
 	check_null_expand(&tree);
 	mtree_destroy(&tree);
-- 
2.43.0