memblock: Check memory add/cap ordering

For memblock_cap_memory_range() to work properly, it should be called
after memory is detected and added to memblock with memblock_add() or
memblock_add_node().  If memblock_cap_memory_range() would be called
before memory is registered, we may silently corrupt memory later
because the crash kernel will see all memory as available.

Print a warning and bail out if ordering is not satisfied.
Suggested-by: Mike Rapoport <rppt@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/aabc5bad008d49f07d542815c6c8d28ec90bb09e.1628672091.git.geert+renesas@glider.be

mm/memblock.c

@@ -1685,6 +1685,11 @@ void __init memblock_cap_memory_range(phys_addr_t base, phys_addr_t size)
 	if (!size)
 		return;
 
+	if (memblock.memory.cnt <= 1) {
+		pr_warn("%s: No memory registered yet\n", __func__);
+		return;
+	}
+
 	ret = memblock_isolate_range(&memblock.memory, base, size,
 						&start_rgn, &end_rgn);
 	if (ret)