arm64: mm: check at build time that PAGE_OFFSET divides the VA space evenly

Commit 8439e62a1561 ("arm64: mm: use bit ops rather than arithmetic in
pa/va translations") changed the boundary check against PAGE_OFFSET from
an arithmetic comparison to a bit test. This means we now silently assume
that PAGE_OFFSET is a power of 2 that divides the kernel virtual address
space into two equal halves. So make that assumption explicit.

Signed-off-by: Ard Biesheuvel <[e-mail]>
Signed-off-by: Catalin Marinas <[e-mail]>

arch/arm64/mm/init.c

···
 	const s64 linear_region_size = -(s64)PAGE_OFFSET;
 
 	/*
+	 * Ensure that the linear region takes up exactly half of the kernel
+	 * virtual address space. This way, we can distinguish a linear address
+	 * from a kernel/module/vmalloc address by testing a single bit.
+	 */
+	BUILD_BUG_ON(linear_region_size != BIT(VA_BITS - 1));
+
+	/*
 	 * Select a suitable value for the base of physical memory.
 	 */
 	memstart_addr = round_down(memblock_start_of_DRAM(),