[PATCH 1/2] mm: thp: avoid calling start_stop_khugepaged() in anon_enabled_store()

[Breno Leitao <leitao@debian.org>]

Writing "never" (or any other value) multiple times to
/sys/kernel/mm/transparent_hugepage/hugepages-*/enabled calls
start_stop_khugepaged() each time, even when nothing actually changed.
This causes set_recommended_min_free_kbytes() to run unconditionally,
which is unnecessary and floods the printk buffer with "raising
min_free_kbytes" messages. Example:

  # for i in $(seq 100); do
  #       echo never > /sys/kernel/mm/transparent_hugepage/enabled
  # done

  # dmesg | grep "min_free_kbytes is not updated" | wc -l
  100

Use test_and_set_bit()/test_and_clear_bit() instead of the plain
variants to detect whether any bit actually flipped, and skip the
start_stop_khugepaged() call entirely when the configuration is
unchanged.

With this patch, redoing the same operation becomes a no-op.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 mm/huge_memory.c | 27 ++++++++++++++-------------
 1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 8e2746ea74adf..9abfb115e9329 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -520,36 +520,37 @@ static ssize_t anon_enabled_store(struct kobject *kobj,
 				  const char *buf, size_t count)
 {
 	int order = to_thpsize(kobj)->order;
+	bool changed = false;
 	ssize_t ret = count;
 
 	if (sysfs_streq(buf, "always")) {
 		spin_lock(&huge_anon_orders_lock);
-		clear_bit(order, &huge_anon_orders_inherit);
-		clear_bit(order, &huge_anon_orders_madvise);
-		set_bit(order, &huge_anon_orders_always);
+		changed = test_and_clear_bit(order, &huge_anon_orders_inherit);
+		changed |= test_and_clear_bit(order, &huge_anon_orders_madvise);
+		changed |= !test_and_set_bit(order, &huge_anon_orders_always);
 		spin_unlock(&huge_anon_orders_lock);
 	} else if (sysfs_streq(buf, "inherit")) {
 		spin_lock(&huge_anon_orders_lock);
-		clear_bit(order, &huge_anon_orders_always);
-		clear_bit(order, &huge_anon_orders_madvise);
-		set_bit(order, &huge_anon_orders_inherit);
+		changed = test_and_clear_bit(order, &huge_anon_orders_always);
+		changed |= test_and_clear_bit(order, &huge_anon_orders_madvise);
+		changed |= !test_and_set_bit(order, &huge_anon_orders_inherit);
 		spin_unlock(&huge_anon_orders_lock);
 	} else if (sysfs_streq(buf, "madvise")) {
 		spin_lock(&huge_anon_orders_lock);
-		clear_bit(order, &huge_anon_orders_always);
-		clear_bit(order, &huge_anon_orders_inherit);
-		set_bit(order, &huge_anon_orders_madvise);
+		changed = test_and_clear_bit(order, &huge_anon_orders_always);
+		changed |= test_and_clear_bit(order, &huge_anon_orders_inherit);
+		changed |= !test_and_set_bit(order, &huge_anon_orders_madvise);
 		spin_unlock(&huge_anon_orders_lock);
 	} else if (sysfs_streq(buf, "never")) {
 		spin_lock(&huge_anon_orders_lock);
-		clear_bit(order, &huge_anon_orders_always);
-		clear_bit(order, &huge_anon_orders_inherit);
-		clear_bit(order, &huge_anon_orders_madvise);
+		changed = test_and_clear_bit(order, &huge_anon_orders_always);
+		changed |= test_and_clear_bit(order, &huge_anon_orders_inherit);
+		changed |= test_and_clear_bit(order, &huge_anon_orders_madvise);
 		spin_unlock(&huge_anon_orders_lock);
 	} else
 		ret = -EINVAL;
 
-	if (ret > 0) {
+	if (ret > 0 && changed) {
 		int err;
 
 		err = start_stop_khugepaged();

-- 
2.47.3