linux-kernel - [PATCH 1/4] x86/mm: Add a few comments

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Date:   Thu, 10 Nov 2022 13:52:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, dave.hansen@...el.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        torvalds@...ux-foundation.org, oliver.sang@...el.com
Subject: [PATCH 1/4] x86/mm: Add a few comments

It's a shame to hide useful comments in Changelogs, add some to the
code.

Shamelessly stolen from commit:

  c40a56a7818c ("x86/mm/init: Remove freed kernel image areas from alias mapping")

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/mm/pat/set_memory.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -219,6 +219,17 @@ within_inclusive(unsigned long addr, uns
 
 #ifdef CONFIG_X86_64
 
+/*
+ * The kernel image is mapped into two places in the virtual address space
+ * (addresses without KASLR, of course):
+ *
+ * 1. The kernel direct map (0xffff880000000000)
+ * 2. The "high kernel map" (0xffffffff81000000)
+ *
+ * We actually execute out of #2. If we get the address of a kernel symbol, it
+ * points to #2, but almost all physical-to-virtual translations point to #1.
+ */
+
 static inline unsigned long highmap_start_pfn(void)
 {
 	return __pa_symbol(_text) >> PAGE_SHIFT;
@@ -1626,6 +1637,9 @@ static int __change_page_attr(struct cpa
 
 static int __change_page_attr_set_clr(struct cpa_data *cpa, int checkalias);
 
+/*
+ * Check the directmap and "high kernel map" 'aliases'.
+ */
 static int cpa_process_alias(struct cpa_data *cpa)
 {
 	struct cpa_data alias_cpa;