On Wed, Apr 28, 2021 at 03:36:02PM +0000, Liam Howlett wrote:

+/*
+ * mas_set_alloc_req() - Set the requested number of allocations.
+ * @mas: the maple state
+ * @count: the number of allocations.
+ *
+ * If @mas->alloc has bit 1 set (0x1) or @mas->alloc is %NULL, then there are no
+ * nodes allocated and @mas->alloc should be set to count << 1 | 1.  If there is
+ * already nodes allocated, then @mas->alloc->request_count stores the request.
+ */
+static inline void mas_set_alloc_req(struct ma_state *mas, unsigned long count)
+{
+	if (!mas->alloc || ((unsigned long)mas->alloc & 0x1)) {
+		if (!count)
+			mas->alloc = NULL;
+		else
+			mas->alloc = (struct maple_alloc *)(((count) << 1U) | 1U);
+		return;
+	}
+
+	mas->alloc->request_count = count;
+}
+
+/*
+ * mas_alloc_req() - get the requested number of allocations.
+ * @mas: The maple state
+ *
+ * The alloc count is either stored directly in @mas, or in
+ * @mas->alloc->request_count if there is at least one node allocated.
+ *
+ * Return: The allocation request count.
+ */
+static inline unsigned int mas_alloc_req(const struct ma_state *mas)
+{
+	if ((unsigned long)mas->alloc & 0x1)
+		return (unsigned long)(mas->alloc) >> 1;
+	else if (mas->alloc)
+		return mas->alloc->request_count;
+	return 0;
+}

I'm confuse.. and the comments fail to eludicate *why* the code is the
way it is, they simply explain exactly what the code does, which I can
already tell from reading the code.

Why can't we have ->request_count unconditionally be the requested
count, and have ->alloc be NULL or not. Why do we play games with low
pointer bits here? AFAICT there's no actual benefit to doing so.