[PATCH 6/12] mm: page migration use the put_new_page whenever necessary

Hugh Dickins
2015-10-19T04:57:23Z

I don't know of any problem from the way it's used in our current tree,
but there is one defect in page migration's custom put_new_page feature.

An unused newpage is expected to be released with the put_new_page(),
but there was one MIGRATEPAGE_SUCCESS (0) path which released it with
putback_lru_page(): which can be very wrong for a custom pool.

Fixed more easily by resetting put_new_page once it won't be needed,
than by adding a further flag to modify the rc test.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
mm/migrate.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)

--- migrat.orig/mm/migrate.c 2015-10-18 17:53:17.579329434 -0700
+++ migrat/mm/migrate.c 2015-10-18 17:53:20.159332371 -0700
@@ -938,10 +938,11 @@ static ICE_noinline int unmap_and_move(n
int force, enum migrate_mode mode,
enum migrate_reason reason)
{
- int rc = 0;
+ int rc = MIGRATEPAGE_SUCCESS;
int *result = NULL;
- struct page *newpage = get_new_page(page, private, &result);
+ struct page *newpage;

+ newpage = get_new_page(page, private, &result);
if (!newpage)
return -ENOMEM;

@@ -955,6 +956,8 @@ static ICE_noinline int unmap_and_move(n
goto out;

rc = __unmap_and_move(page, newpage, force, mode);
+ if (rc == MIGRATEPAGE_SUCCESS)
+ put_new_page = NULL;

out:
if (rc != -EAGAIN) {
@@ -981,7 +984,7 @@ out:
* it. Otherwise, putback_lru_page() will drop the reference grabbed
* during isolation.
*/
- if (rc != MIGRATEPAGE_SUCCESS && put_new_page) {
+ if (put_new_page) {
ClearPageSwapBacked(newpage);
put_new_page(newpage, private);
} else if (unlikely(__is_movable_balloon_page(newpage))) {
@@ -1022,7 +1025,7 @@ static int unmap_and_move_huge_page(new_
struct page *hpage, int force,
enum migrate_mode mode)
{
- int rc = 0;
+ int rc = -EAGAIN;
int *result = NULL;
int page_was_mapped = 0;
struct page *new_hpage;
@@ -1044,8 +1047,6 @@ static int unmap_and_move_huge_page(new_
if (!new_hpage)
return -ENOMEM;

- rc = -EAGAIN;
-
if (!trylock_page(hpage)) {
if (!force || mode != MIGRATE_SYNC)
goto out;
@@ -1070,8 +1071,10 @@ static int unmap_and_move_huge_page(new_
if (anon_vma)
put_anon_vma(anon_vma);

- if (rc == MIGRATEPAGE_SUCCESS)
+ if (rc == MIGRATEPAGE_SUCCESS) {
hugetlb_cgroup_migrate(hpage, new_hpage);
+ put_new_page = NULL;
+ }

unlock_page(hpage);
out:
@@ -1083,7 +1086,7 @@ out:
* it. Otherwise, put_page() will drop the reference grabbed during
* isolation.
*/
- if (rc != MIGRATEPAGE_SUCCESS && put_new_page)
+ if (put_new_page)
put_new_page(new_hpage, private);
else
putback_active_hugepage(new_hpage);