[PATCH v3 6/6] mm: add sysfs entry to disable splitting underutilized THPs

[Usama Arif <usamaarif642@xxxxxxxxx>]

If disabled, THPs faulted in or collapsed will not be added to
_deferred_list, and therefore won't be considered for splitting under
memory pressure if underutilized.

Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
---
 mm/huge_memory.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 6b32b2d4ab1e..b4d72479330d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -74,6 +74,7 @@ static unsigned long deferred_split_count(struct shrinker *shrink,
 					  struct shrink_control *sc);
 static unsigned long deferred_split_scan(struct shrinker *shrink,
 					 struct shrink_control *sc);
+static bool split_underutilized_thp = true;
 
 static atomic_t huge_zero_refcount;
 struct folio *huge_zero_folio __read_mostly;
@@ -439,6 +440,27 @@ static ssize_t hpage_pmd_size_show(struct kobject *kobj,
 static struct kobj_attribute hpage_pmd_size_attr =
 	__ATTR_RO(hpage_pmd_size);
 
+static ssize_t split_underutilized_thp_show(struct kobject *kobj,
+			    struct kobj_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%d\n", split_underutilized_thp);
+}
+
+static ssize_t split_underutilized_thp_store(struct kobject *kobj,
+			     struct kobj_attribute *attr,
+			     const char *buf, size_t count)
+{
+	int err = kstrtobool(buf, &split_underutilized_thp);
+
+	if (err < 0)
+		return err;
+
+	return count;
+}
+
+static struct kobj_attribute split_underutilized_thp_attr = __ATTR(
+	thp_low_util_shrinker, 0644, split_underutilized_thp_show, split_underutilized_thp_store);
+
 static struct attribute *hugepage_attr[] = {
 	&enabled_attr.attr,
 	&defrag_attr.attr,
@@ -447,6 +469,7 @@ static struct attribute *hugepage_attr[] = {
 #ifdef CONFIG_SHMEM
 	&shmem_enabled_attr.attr,
 #endif
+	&split_underutilized_thp_attr.attr,
 	NULL,
 };
 
@@ -3475,6 +3498,9 @@ void deferred_split_folio(struct folio *folio, bool partially_mapped)
 	if (folio_order(folio) <= 1)
 		return;
 
+	if (!partially_mapped && !split_underutilized_thp)
+		return;
+
 	/*
 	 * The try_to_unmap() in page reclaim path might reach here too,
 	 * this may cause a race condition to corrupt deferred split queue.
-- 
2.43.5