s390/mm,ptdump: hold cpa mutex while walking for kernel page table dump

This is currently only preventing that outdated information is
provided to user space. A concurrent split of huge/large pages does
modify the kernel page tables, however either the huge/large mapping
is reported or the split area is being walked.

This "fixes" also only a potential future bug, since split pages could
also be merged again if page permissions are the same for larger
memory areas.

Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/set_memory.h

@@ -2,6 +2,10 @@
 #ifndef _ASMS390_SET_MEMORY_H
 #define _ASMS390_SET_MEMORY_H
 
+#include <linux/mutex.h>
+
+extern struct mutex cpa_mutex;
+
 #define SET_MEMORY_RO	1UL
 #define SET_MEMORY_RW	2UL
 #define SET_MEMORY_NX	4UL

arch/s390/mm/dump_pagetables.c

@@ -1,4 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0
+#include <linux/set_memory.h>
 #include <linux/ptdump.h>
 #include <linux/seq_file.h>
 #include <linux/debugfs.h>
@@ -130,7 +131,9 @@ static int ptdump_show(struct seq_file *m, void *v)
 	};
 
 	get_online_mems();
+	mutex_lock(&cpa_mutex);
 	ptdump_walk_pgd(&st.ptdump, &init_mm, NULL);
+	mutex_unlock(&cpa_mutex);
 	put_online_mems();
 	return 0;
 }

arch/s390/mm/pageattr.c

@@ -278,7 +278,7 @@ static int walk_p4d_level(pgd_t *pgd, unsigned long addr, unsigned long end,
 	return rc;
 }
 
-static DEFINE_MUTEX(cpa_mutex);
+DEFINE_MUTEX(cpa_mutex);
 
 static int change_page_attr(unsigned long addr, unsigned long end,
 			    unsigned long flags)