Subject: x86/mm/ptdump: Fix soft lockup in page table walker

From: Andrey Ryabinin <[email-masked]>
Date: Fri, 10 Feb 2017 12:54:05 +0300

commit 146fbb766934dc003fcbf755b519acef683576bf upstream.

CONFIG_KASAN=y needs a lot of virtual memory mapped for its shadow.
In that case ptdump_walk_pgd_level_core() takes a lot of time to
walk across all page tables and doing this without
a rescheduling causes soft lockups:

 NMI watchdog: BUG: soft lockup - CPU#3 stuck for 23s! [swapper/0:1]
 ...
 Call Trace:
  ptdump_walk_pgd_level_core+0x40c/0x550
  ptdump_walk_pgd_level_checkwx+0x17/0x20
  mark_rodata_ro+0x13b/0x150
  kernel_init+0x2f/0x120
  ret_from_fork+0x2c/0x40

I guess that this issue might arise even without KASAN on huge machines
with several terabytes of RAM.

Stick cond_resched() in pgd loop to fix this.

Reported-by: Tobias Regnery <[email-masked]>
Signed-off-by: Andrey Ryabinin <[email-masked]>
Cc: [email-masked]
Cc: Alexander Potapenko <[email-masked]>
Cc: "Paul E . McKenney" <[email-masked]>
Cc: Dmitry Vyukov <[email-masked]>
Cc: [email-masked]
Link: http://lkml.kernel.org/r/[email-masked]
Signed-off-by: Thomas Gleixner <[email-masked]>
[bwh: Backported to 4.4: adjust context]
Signed-off-by: Ben Hutchings <[email-masked]>
---
 arch/x86/mm/dump_pagetables.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
index 0f1c6fc3ddd8..47770ccab6d7 100644
--- a/arch/x86/mm/dump_pagetables.c
+++ b/arch/x86/mm/dump_pagetables.c
@@ -15,6 +15,7 @@
 #include <linux/debugfs.h>
 #include <linux/mm.h>
 #include <linux/module.h>
+#include <linux/sched.h>
 #include <linux/seq_file.h>
 
 #include <asm/pgtable.h>
@@ -407,6 +408,7 @@ static void ptdump_walk_pgd_level_core(struct seq_file *m, pgd_t *pgd,
 		} else
 			note_page(m, &st, __pgprot(0), 1);
 
+		cond_resched();
 		start++;
 	}