[PATCH v6 3/4] mm: huge_memory: refactor enabled_store() with change_enabled()

[Breno Leitao <leitao@debian.org>]

Refactor enabled_store() to use a new change_enabled() helper.
Introduce a separate enum global_enabled_mode and
global_enabled_mode_strings[], mirroring the anon_enabled_mode
pattern from the previous commit.

A separate enum is necessary because the global THP setting does
not support "inherit", only "always", "madvise", and "never".
Reusing anon_enabled_mode would leave a NULL gap in the string
array, causing sysfs_match_string() to stop early and fail to
match entries after the gap.

The helper uses the same loop pattern as set_anon_enabled_mode(),
iterating over an array of flag bit positions and using
__test_and_set_bit()/__test_and_clear_bit() to track whether the state
actually changed.

Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
---
 mm/huge_memory.c | 63 ++++++++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 48 insertions(+), 15 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f6af90e6cf05d..8949393de7a51 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -330,30 +330,63 @@ static const char * const anon_enabled_mode_strings[] = {
 	[ANON_ENABLED_NEVER]	= "never",
 };
 
+enum global_enabled_mode {
+	GLOBAL_ENABLED_ALWAYS	= 0,
+	GLOBAL_ENABLED_MADVISE	= 1,
+	GLOBAL_ENABLED_NEVER	= 2,
+};
+
+static const char * const global_enabled_mode_strings[] = {
+	[GLOBAL_ENABLED_ALWAYS]		= "always",
+	[GLOBAL_ENABLED_MADVISE]	= "madvise",
+	[GLOBAL_ENABLED_NEVER]		= "never",
+};
+
+static bool set_global_enabled_mode(enum global_enabled_mode mode)
+{
+	static const unsigned long thp_flags[] = {
+		TRANSPARENT_HUGEPAGE_FLAG,
+		TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
+	};
+	enum global_enabled_mode m;
+	bool changed = false;
+
+	for (m = 0; m < ARRAY_SIZE(thp_flags); m++) {
+		if (m == mode)
+			changed |= !__test_and_set_bit(thp_flags[m],
+						       &transparent_hugepage_flags);
+		else
+			changed |= __test_and_clear_bit(thp_flags[m],
+							&transparent_hugepage_flags);
+	}
+
+	return changed;
+}
+
 static ssize_t enabled_store(struct kobject *kobj,
 			     struct kobj_attribute *attr,
 			     const char *buf, size_t count)
 {
-	ssize_t ret = count;
+	int mode;
 
-	if (sysfs_streq(buf, "always")) {
-		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
-		set_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
-	} else if (sysfs_streq(buf, "madvise")) {
-		clear_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
-		set_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
-	} else if (sysfs_streq(buf, "never")) {
-		clear_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags);
-		clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags);
-	} else
-		ret = -EINVAL;
+	mode = sysfs_match_string(global_enabled_mode_strings, buf);
+	if (mode < 0)
+		return -EINVAL;
 
-	if (ret > 0) {
+	if (set_global_enabled_mode(mode)) {
 		int err = start_stop_khugepaged();
+
 		if (err)
-			ret = err;
+			return err;
+	} else {
+		/*
+		 * Recalculate watermarks even when the mode didn't
+		 * change, as the previous code always called
+		 * start_stop_khugepaged() which does this internally.
+		 */
+		set_recommended_min_free_kbytes();
 	}
-	return ret;
+	return count;
 }
 
 static struct kobj_attribute enabled_attr = __ATTR_RW(enabled);

-- 
2.52.0