[tip:x86/mm] x86/mm: Add clarifying comments for user addr space

[tip-bot for Dave Hansen <tipbot@zytor.com>]

Commit-ID:  5b0c2cac54d44ea40d5c5aec7cbf14353b2a903e
Gitweb:     https://git.kernel.org/tip/5b0c2cac54d44ea40d5c5aec7cbf14353b2a903e
Author:     Dave Hansen <dave.hansen@linux.intel.com>
AuthorDate: Fri, 28 Sep 2018 09:02:25 -0700
Committer:  Peter Zijlstra <peterz@infradead.org>
CommitDate: Tue, 9 Oct 2018 16:51:16 +0200

x86/mm: Add clarifying comments for user addr space

The SMAP and Reserved checking do not have nice comments.  Add
some to clarify and make it match everything else.

Cc: x86@kernel.org
Cc: Jann Horn <jannh@google.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180928160225.FFD44B8D@viggo.jf.intel.com
---
 arch/x86/mm/fault.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 0d1f5d39fc63..1d838701a5f7 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1276,9 +1276,17 @@ void do_user_addr_fault(struct pt_regs *regs,
 	if (unlikely(kprobes_fault(regs)))
 		return;
 
+	/*
+	 * Reserved bits are never expected to be set on
+	 * entries in the user portion of the page tables.
+	 */
 	if (unlikely(hw_error_code & X86_PF_RSVD))
 		pgtable_bad(regs, hw_error_code, address);
 
+	/*
+	 * Check for invalid kernel (supervisor) access to user
+	 * pages in the user address space.
+	 */
 	if (unlikely(smap_violation(hw_error_code, regs))) {
 		bad_area_nosemaphore(regs, hw_error_code, address, NULL);
 		return;