[PATCH v2] drop_caches: re-enable message after disabling

[Ruiwu Chen <rwchen404@gmail.com>]

When 'echo 4 > /proc/sys/vm/drop_caches' the message is disabled,
but there is no interface to enable the message, only by restarting
the way, so add the 'echo 0 > /proc/sys/vm/drop_caches' way to
enabled the message again.

Signed-off-by: Ruiwu Chen <rwchen404@gmail.com>
---
v2: - updated Documentation/ to note this new API.
    - renamed the variable.
    - rebase this on top of sysctl-next [1].
[1] https://git.kernel.org/pub/scm/linux/kernel/git/sysctl/sysctl.git/log/?h=sysctl-next

 Documentation/admin-guide/sysctl/vm.rst | 11 ++++++++++-
 fs/drop_caches.c                        | 11 +++++++----
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index f48eaa98d22d..ef73d36e8b84 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -266,7 +266,16 @@ used::
 	cat (1234): drop_caches: 3
 
 These are informational only.  They do not mean that anything is wrong
-with your system.  To disable them, echo 4 (bit 2) into drop_caches.
+with your system.
+
+To disable informational::
+
+	echo 4 > /proc/sys/vm/drop_caches
+
+To enable informational::
+
+	echo 0 > /proc/sys/vm/drop_caches
+
 
 enable_soft_offline
 ===================
diff --git a/fs/drop_caches.c b/fs/drop_caches.c
index 019a8b4eaaf9..a49af7023886 100644
--- a/fs/drop_caches.c
+++ b/fs/drop_caches.c
@@ -57,7 +57,7 @@ static int drop_caches_sysctl_handler(const struct ctl_table *table, int write,
 	if (ret)
 		return ret;
 	if (write) {
-		static int stfu;
+		static bool silent;
 
 		if (sysctl_drop_caches & 1) {
 			lru_add_drain_all();
@@ -68,12 +68,15 @@ static int drop_caches_sysctl_handler(const struct ctl_table *table, int write,
 			drop_slab();
 			count_vm_event(DROP_SLAB);
 		}
-		if (!stfu) {
+		if (!silent) {
 			pr_info("%s (%d): drop_caches: %d\n",
 				current->comm, task_pid_nr(current),
 				sysctl_drop_caches);
 		}
-		stfu |= sysctl_drop_caches & 4;
+		if (sysctl_drop_caches == 0)
+			silent = true;
+		else if (sysctl_drop_caches == 4)
+			silent = false;
 	}
 	return 0;
 }
@@ -85,7 +88,7 @@ static const struct ctl_table drop_caches_table[] = {
 		.maxlen		= sizeof(int),
 		.mode		= 0200,
 		.proc_handler	= drop_caches_sysctl_handler,
-		.extra1		= SYSCTL_ONE,
+		.extra1		= SYSCTL_ZERO,
 		.extra2		= SYSCTL_FOUR,
 	},
 };
-- 
2.27.0