maple_tree: refine mas_store_root() on storing NULL

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

Storing NULLs over the entire tree may result in a node being used to
store a single range.  Further stores of NULL may cause the node and
tree to be corrupt and cause incorrect behaviour.  Fixing the store to
the root null fixes the issue by ensuring that a range of 0 - ULONG_MAX
results in an empty tree.

Users of the tree may experience incorrect values returned if the tree
was expanded to store values, then overwritten by all NULLS, then
continued to store NULLs over the empty area.

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

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 54a611b ("Maple Tree: add new data structure")
Signed-off-by: Wei Yang <[email protected]>
Reviewed-by: Liam R. Howlett <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Sidhartha Kumar <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: RichardWeiYang

lib/maple_tree.c

@@ -3447,9 +3447,20 @@ 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);