[PATCH v4 4/5] maple_tree: refine mas_store_root() on storing NULL

[Wei Yang <richard.weiyang@gmail.com>]

Currently, when storing NULL on mas_store_root(), the behavior could be
improved.

For example possible cases are:

  * store NULL at any range result a new node
  * store NULL at range [m, n] where m > 0 to a single entry tree result
    a new node with range [m, n] set to NULL
  * store NULL at range [m, n] where m > 0 to an empty tree result
    consecutive NULL slot 
  * it allows for multiple NULL entries by expanding root to
    store NULLs to an empty tree

This patch tries to improve in:

  * memory efficient by setting to empty tree instead of using a node
  * remove the possibility of consecutive NULL slot which will prohibit
    extended null in later operation

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
CC: Liam R. Howlett <Liam.Howlett@Oracle.com>
CC: Sidhartha Kumar <sidhartha.kumar@oracle.com>
CC: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

---
v3: move change into mas_store_root()
v4: add a comment and simplify the logic a little
    adjust the change log a little
---
 lib/maple_tree.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 2226e77c00cb..1205a5208cfe 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -3439,9 +3439,20 @@ static inline void mas_root_expand(struct ma_state *mas, void *entry)
 	return;
 }
 
+/*
+ * mas_store_root() - Storing value into root.
+ * @mas: The maple state
+ * @entry: The entry to store.
+ *
+ * There is no root node now and we are storing a value into the root - this
+ * function either assigns the pointer or expands into a node.
+ */
 static inline void mas_store_root(struct ma_state *mas, void *entry)
 {
-	if (likely((mas->last != 0) || (mas->index != 0)))
+	if (!entry) {
+		if (!mas->index)
+			rcu_assign_pointer(mas->tree->ma_root, NULL);
+	} else if (likely((mas->last != 0) || (mas->index != 0)))
 		mas_root_expand(mas, entry);
 	else if (((unsigned long) (entry) & 3) == 2)
 		mas_root_expand(mas, entry);
-- 
2.34.1