Demote page functionality will split a huge page into a number of huge
pages of a smaller size.  For example, on x86 a 1GB huge page can be
demoted into 512 2M huge pages.  Demotion is done 'in place' by simply
splitting the huge page.

Added '*_for_demote' wrappers for remove_hugetlb_page,
destroy_compound_gigantic_page and prep_compound_gigantic_page for use
by demote code.

Signed-off-by: Mike Kravetz <redacted>
---
  mm/hugetlb.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++++---
  1 file changed, 75 insertions(+), 4 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 2317d411243d..ab7bd0434057 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c

@@ -1260,7 +1260,7 @@ static int hstate_next_node_to_free(struct hstate *h, nodemask_t *nodes_allowed)
  		((node = hstate_next_node_to_free(hs, mask)) || 1);	\
  		nr_nodes--)
  
-#ifdef CONFIG_ARCH_HAS_GIGANTIC_PAGE
+/* used to demote non-gigantic_huge pages as well */
  static void __destroy_compound_gigantic_page(struct page *page,
  					unsigned int order, bool demote)
  {

@@ -1283,6 +1283,13 @@ static void __destroy_compound_gigantic_page(struct page *page,
  	__ClearPageHead(page);
  }
  
+static void destroy_compound_gigantic_page_for_demote(struct page *page,
+					unsigned int order)
+{
+	__destroy_compound_gigantic_page(page, order, true);
+}
+
+#ifdef CONFIG_ARCH_HAS_GIGANTIC_PAGE
  static void destroy_compound_gigantic_page(struct page *page,
  					unsigned int order)
  {

@@ -1428,6 +1435,12 @@ static void remove_hugetlb_page(struct hstate *h, struct page *page,
  	__remove_hugetlb_page(h, page, adjust_surplus, false);
  }
  
+static void remove_hugetlb_page_for_demote(struct hstate *h, struct page *page,
+							bool adjust_surplus)
+{
+	__remove_hugetlb_page(h, page, adjust_surplus, true);
+}
+
  static void add_hugetlb_page(struct hstate *h, struct page *page,
  			     bool adjust_surplus)
  {

@@ -1777,6 +1790,12 @@ static bool prep_compound_gigantic_page(struct page *page, unsigned int order)
  	return __prep_compound_gigantic_page(page, order, false);
  }
  
+static bool prep_compound_gigantic_page_for_demote(struct page *page,
+							unsigned int order)
+{
+	return __prep_compound_gigantic_page(page, order, true);
+}
+
  /*
   * PageHuge() only returns true for hugetlbfs pages, but not for normal or
   * transparent huge pages.  See the PageTransHuge() documentation for more

@@ -3298,9 +3317,55 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
  	return 0;
  }
  
+static int demote_free_huge_page(struct hstate *h, struct page *page)
+{
+	int i, nid = page_to_nid(page);
+	struct hstate *target_hstate;
+	bool cma_page = HPageCma(page);
+
+	target_hstate = size_to_hstate(PAGE_SIZE << h->demote_order);
+
+	remove_hugetlb_page_for_demote(h, page, false);
+	spin_unlock_irq(&hugetlb_lock);
+
+	if (alloc_huge_page_vmemmap(h, page)) {
+		/* Allocation of vmemmmap failed, we can not demote page */
+		spin_lock_irq(&hugetlb_lock);
+		set_page_refcounted(page);
+		add_hugetlb_page(h, page, false);