[PATCH] swap: add a limit for readahead page-cluster value

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

Currenty there is no upper limit for /proc/sys/vm/page-cluster, and it's
a bit shift value, so it could result in overflow of the 32-bit integer.
Add a reasonable upper limit for it, read-in at most 2**31 pages, which
is a large enough value for readahead.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 include/linux/mm.h | 1 +
 kernel/sysctl.c    | 1 +
 mm/swap.c          | 3 ++-
 3 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index d42536ce1dab4..5512f3b188fab 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -74,6 +74,7 @@ static inline void totalram_pages_add(long count)
 
 extern void * high_memory;
 extern int page_cluster;
+extern const int page_cluster_max;
 
 #ifdef CONFIG_SYSCTL
 extern int sysctl_legacy_va_layout;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 188c305aeb8b7..71a4350ac601b 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2125,6 +2125,7 @@ static struct ctl_table vm_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec_minmax,
 		.extra1		= SYSCTL_ZERO,
+		.extra2		= (void *)&page_cluster_max,
 	},
 	{
 		.procname	= "dirtytime_expire_seconds",
diff --git a/mm/swap.c b/mm/swap.c
index 955930f41d20c..2ab33c82cb062 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -43,8 +43,9 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/pagemap.h>
 
-/* How many pages do we try to swap or page in/out together? */
+/* How many pages do we try to swap or page in/out together? As a power of 2 */
 int page_cluster;
+const int page_cluster_max = 31;
 
 /* Protecting only lru_rotate.fbatch which requires disabling interrupts */
 struct lru_rotate {
-- 
2.35.2