[PATCH 2/4] mm, migrate: allocation new page lazyily in unmap_and_move()

[Joonsoo Kim]

We don't need a new page and then go out immediately if some condition
is met. Allocation has overhead in comparison with some condition check,
so allocating lazyily is preferable solution.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

diff --git a/mm/migrate.c b/mm/migrate.c
index 6f0c244..86db87e 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -864,10 +864,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 {
 	int rc = 0;
 	int *result = NULL;
-	struct page *newpage = get_new_page(page, private, &result);
-
-	if (!newpage)
-		return -ENOMEM;
+	struct page *newpage = NULL;
 
 	if (page_count(page) == 1) {
 		/* page was freed from under us. So we are done. */
@@ -878,6 +875,10 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 		if (unlikely(split_huge_page(page)))
 			goto out;
 
+	newpage = get_new_page(page, private, &result);
+	if (!newpage)
+		return -ENOMEM;
+
 	rc = __unmap_and_move(page, newpage, force, mode);
 
 	if (unlikely(rc == MIGRATEPAGE_BALLOON_SUCCESS)) {
@@ -908,7 +909,8 @@ out:
 	 * Move the new page to the LRU. If migration was not successful
 	 * then this will free the page.
 	 */
-	putback_lru_page(newpage);
+	if (newpage)
+		putback_lru_page(newpage);
 	if (result) {
 		if (rc)
 			*result = rc;
-- 
1.7.9.5

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