[RFC 2/7] mm: introduce split_page_by_order

[Minchan Kim <minchan@xxxxxxxxxx>]

This patch introduces split_page_by_order to support splitting
a high-order page into group of smaller high-order pages and
use it in split_map_pages for supporting upcoming high-order
bulk operation.

This patch shouldn't change any behavior.

Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 include/linux/mm.h |  2 ++
 mm/compaction.c    |  2 +-
 mm/page_alloc.c    | 27 +++++++++++++++++++--------
 3 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8ab941cf73f4..9a51abbe8625 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -849,6 +849,8 @@ void __put_page(struct page *page);
 
 void put_pages_list(struct list_head *pages);
 
+void split_page_by_order(struct page *page, unsigned int order,
+			unsigned int new_order);
 void split_page(struct page *page, unsigned int order);
 
 /*
diff --git a/mm/compaction.c b/mm/compaction.c
index 176dcded298e..f31799a841f2 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -98,7 +98,7 @@ static void split_map_pages(struct list_head *list)
 
 		post_alloc_hook(page, order, __GFP_MOVABLE);
 		if (order)
-			split_page(page, order);
+			split_page_by_order(page, order, 0);
 
 		for (i = 0; i < nr_pages; i++) {
 			list_add(&page->lru, &tmp_list);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 8ce30cc50577..4caab47377a7 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3188,6 +3188,24 @@ void free_unref_page_list(struct list_head *list)
 	local_irq_restore(flags);
 }
 
+/*
+ * split_page_by_order takes a non-compound higher-order page, and splits
+ * it into n (1 << (order - new_order)) sub-order pages: page[0..n]
+ * Each sub-page must be freed individually.
+ */
+void split_page_by_order(struct page *page, unsigned int order,
+			unsigned int new_order)
+{
+	int i;
+
+	VM_BUG_ON_PAGE(PageCompound(page), page);
+	VM_BUG_ON_PAGE(!page_count(page), page);
+
+	for (i = 1; i < (1 << (order - new_order)); i++)
+		set_page_refcounted(page + i * (1 << new_order));
+	split_page_owner(page, order, new_order);
+}
+
 /*
  * split_page takes a non-compound higher-order page, and splits it into
  * n (1<<order) sub-pages: page[0..n]
@@ -3198,14 +3216,7 @@ void free_unref_page_list(struct list_head *list)
  */
 void split_page(struct page *page, unsigned int order)
 {
-	int i;
-
-	VM_BUG_ON_PAGE(PageCompound(page), page);
-	VM_BUG_ON_PAGE(!page_count(page), page);
-
-	for (i = 1; i < (1 << order); i++)
-		set_page_refcounted(page + i);
-	split_page_owner(page, order, 0);
+	split_page_by_order(page, order, 0);
 }
 EXPORT_SYMBOL_GPL(split_page);
 
-- 
2.28.0.220.ged08abb693-goog