+/*
+ * Log2 of the upper bound of the size of a struct page. Used for sizing
+ * the vmemmap region only, does not affect actual memory footprint.
+ * We don't use sizeof(struct page) directly since taking its size here
+ * requires its definition to be available at this point in the inclusion
+ * chain, and it may not be a power of 2 in the first place.
+ */
+#define STRUCT_PAGE_MAX_SHIFT	6

I know this is copied from arm64, but wouldn't this be a good time
to move this next to the struct page defintion?

Also this:

arch/arm64/mm/init.c:   BUILD_BUG_ON(sizeof(struct page) > (1 << STRUCT_PAGE_MAX_SHIFT));

should move to comment code (or would have to be duplicated for riscv)

+#define VMEMMAP_SIZE	(UL(1) << (CONFIG_VA_BITS - PAGE_SHIFT - 1 + \
+				   STRUCT_PAGE_MAX_SHIFT))

Might be more readable with a another define, and without abuse of the
horrible UL macro:

#define VMEMMAP_SHIFT \
	(CONFIG_VA_BITS - PAGE_SHIFT - 1 + STRUCT_PAGE_MAX_SHIFT)
#define VMEMMAP_SIZE	(1UL << VMEMMAP_SHIFT)

+#define VMEMMAP_END	(VMALLOC_START - 1)
+#define VMEMMAP_START	(VMALLOC_START - VMEMMAP_SIZE)
+
+#define vmemmap		((struct page *)VMEMMAP_START)

This could also use some comments..

For potentially wide-spanning ifdefs like inclusion headers it always
is nice to have a comment with the symbol on the endif line.