On Thu, 21 Dec 2023 17:35:22 +0000
Vincent Donnefort [off-list ref] wrote:

The kernel developers have agreed to allow loop variables to be declared in
loops. This will simplify these macros:

+#define subbuf_page(off, start) \
+	virt_to_page((void *)(start + (off << PAGE_SHIFT)))
+
+#define foreach_subbuf_page(off, sub_order, start, page)	\
+	for (off = 0, page = subbuf_page(0, start);		\
+	     off < (1 << sub_order);				\
+	     off++, page = subbuf_page(off, start))

#define foreach_subbuf_page(sub_order, start, page)		\
	for (int __off = 0, page = subbuf_page(0, (start));	\
	     __off < (1 << (sub_order));			\
	     __off++, page = subbuf_page(__off, (start)))

And parameters as r-values should always be wrapped in parenthesis.

+
+static inline void subbuf_map_prepare(unsigned long subbuf_start, int order)
+{
+	struct page *page;
+	int subbuf_off;

Then you can remove the "subbuf_off".

+
+	/*
+	 * When allocating order > 0 pages, only the first struct page has a
+	 * refcount > 1. Increasing the refcount here ensures none of the struct
+	 * page composing the sub-buffer is freeed when the mapping is closed.

Nice catch by the way ;-)

-- Steve

+	 */
+	foreach_subbuf_page(subbuf_off, order, subbuf_start, page)
+		page_ref_inc(page);
+}
+
+static inline void subbuf_unmap(unsigned long subbuf_start, int order)
+{
+	struct page *page;
+	int subbuf_off;
+
+	foreach_subbuf_page(subbuf_off, order, subbuf_start, page) {
+		page_ref_dec(page);
+		page->mapping = NULL;
+	}
+}
+