[PATCH] mm: implement "memory.oops_if_bad_pte=1" boot option

[Alexey Dobriyan <adobriyan@gmail.com>]

Implement

	memory.oops_if_bad_pte=1

boot option which oopses the machine instead of dreadful

	BUG: Bad page map in process

message.

This is intended
for people who want to panic at the slightest provocation and
for people who ruled out hardware problems which in turn means that
delaying vmcore collection is counter-productive.

Linux doesn't (never?) panicked on PTE corruption and even implemented
ratelimited version of the message meaning it can go for minutes and
even hours without anyone noticing which is exactly the opposite of what
should be done to facilitate debugging.

Not enabled by default.

Not advertised.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 mm/memory.c                                 | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index b0cda5aab398..90b92b312802 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -42,6 +42,7 @@
 #include <linux/kernel_stat.h>
 #include <linux/mm.h>
 #include <linux/mm_inline.h>
+#include <linux/moduleparam.h>
 #include <linux/sched/mm.h>
 #include <linux/sched/numa_balancing.h>
 #include <linux/sched/task.h>
@@ -480,6 +481,13 @@ static inline void add_mm_rss_vec(struct mm_struct *mm, int *rss)
 			add_mm_counter(mm, i, rss[i]);
 }
 
+/*
+ * Oops instead of printing "Bad page map in process" message and
+ * trying to continue.
+ */
+static bool oops_if_bad_pte __ro_after_init = false;
+module_param(oops_if_bad_pte, bool, 0444);
+
 /*
  * This function is called to print an error when a bad pte
  * is found. For example, we might have a PFN-mapped pte in
@@ -490,6 +498,13 @@ static inline void add_mm_rss_vec(struct mm_struct *mm, int *rss)
 static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr,
 			  pte_t pte, struct page *page)
 {
+	/*
+	 * This line is a formality to collect vmcore ASAP. Real bug
+	 * (hardware or software) happened earlier, current registers and
+	 * backtrace aren't interesting.
+	 */
+	BUG_ON(oops_if_bad_pte);
+
 	pgd_t *pgd = pgd_offset(vma->vm_mm, addr);
 	p4d_t *p4d = p4d_offset(pgd, addr);
 	pud_t *pud = pud_offset(p4d, addr);
-- 
2.49.0