x86-64: add comment for RODATA large page retainment

Add a comment explaining why RODATA is aligned to 2 MB.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 14763790e4156d6ad922b46671e758e584ea6625..fd2dabec1dffd982a22f16a13b34a39734546e8f 100644 (file)
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/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                             \