x86-64: add comment for RODATA large page retainment

Add a comment explaining why RODATA is aligned to 2 MB.
Signed-off-by: NSuresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>

arch/x86/kernel/vmlinux.lds.S

@@ -42,7 +42,18 @@ jiffies_64 = jiffies;
 #endif
 
 #if defined(CONFIG_X86_64) && defined(CONFIG_DEBUG_RODATA)
-
+/*
+ * On 64-bit, align RODATA to 2MB so that even with CONFIG_DEBUG_RODATA
+ * we retain large page mappings for boundaries spanning kernel text, rodata
+ * and data sections.
+ *
+ * However, kernel identity mappings will have different RWX permissions
+ * to the pages mapping to text and to the pages padding (which are freed) the
+ * text section. Hence kernel identity mappings will be broken to smaller
+ * pages. For 64-bit, kernel text and kernel identity mappings are different,
+ * so we can enable protection checks that come with CONFIG_DEBUG_RODATA,
+ * as well as retain 2MB large page mappings for kernel text.
+ */
 #define X64_ALIGN_DEBUG_RODATA_BEGIN	. = ALIGN(HPAGE_SIZE);
 
 #define X64_ALIGN_DEBUG_RODATA_END				\