[PATCH] mm, compaction: Use free lists to quickly locate a migration source -fix

[Mel Gorman]

Vlastimil correctly pointed out that when a fast search fails and cc->migrate_pfn
is reinitialised to the lowest PFN found that the caller does not use the updated
PFN.

He also pointed out that there is an inconsistency between
move_freelist_head and move_freelist_tail. This patch adds a new helper
and uses it in move_freelist_tail so that list manipulations are avoided
if the first list item traversed is a suitable migration source. The
end result will be that the helpers should be symmetrical and it's been
confirmed that the scan rates are slightly improved as a result of the
fix but not enough to rewrite the changelogs.

This is a fix for the mmotm patch
mm-compaction-use-free-lists-to-quickly-locate-a-migration-source.patch . It's
been provided as a combined patch as the first patch is not picked up at the
time of writing and a rolled up patch is less likely to fall through the cracks.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/i915/i915_utils.h |  6 ------
 include/linux/list.h              | 11 +++++++++++
 mm/compaction.c                   | 10 ++++++----
 3 files changed, 17 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_utils.h b/drivers/gpu/drm/i915/i915_utils.h
index 9726df37c4c4..540e20eb032c 100644
--- a/drivers/gpu/drm/i915/i915_utils.h
+++ b/drivers/gpu/drm/i915/i915_utils.h
@@ -123,12 +123,6 @@ static inline u64 ptr_to_u64(const void *ptr)
 
 #include <linux/list.h>
 
-static inline int list_is_first(const struct list_head *list,
-				const struct list_head *head)
-{
-	return head->next == list;
-}
-
 static inline void __list_del_many(struct list_head *head,
 				   struct list_head *first)
 {
diff --git a/include/linux/list.h b/include/linux/list.h
index edb7628e46ed..79626b5ab36c 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -206,6 +206,17 @@ static inline void list_bulk_move_tail(struct list_head *head,
 	head->prev = last;
 }
 
+/**
+ * list_is_first -- tests whether @ list is the first entry in list @head
+ * @list: the entry to test
+ * @head: the head of the list
+ */
+static inline int list_is_first(const struct list_head *list,
+					const struct list_head *head)
+{
+	return list->prev == head;
+}
+
 /**
  * list_is_last - tests whether @list is the last entry in list @head
  * @list: the entry to test
diff --git a/mm/compaction.c b/mm/compaction.c
index 92d10eb3d1c7..55f7ab142af2 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1062,7 +1062,7 @@ move_freelist_tail(struct list_head *freelist, struct page *freepage)
 {
 	LIST_HEAD(sublist);
 
-	if (!list_is_last(freelist, &freepage->lru)) {
+	if (!list_is_first(freelist, &freepage->lru)) {
 		list_cut_position(&sublist, freelist, &freepage->lru);
 		if (!list_empty(&sublist))
 			list_splice_tail(&sublist, freelist);
@@ -1238,14 +1238,16 @@ update_fast_start_pfn(struct compact_control *cc, unsigned long pfn)
 	cc->fast_start_pfn = min(cc->fast_start_pfn, pfn);
 }
 
-static inline void
+static inline unsigned long
 reinit_migrate_pfn(struct compact_control *cc)
 {
 	if (!cc->fast_start_pfn || cc->fast_start_pfn == ULONG_MAX)
-		return;
+		return cc->migrate_pfn;
 
 	cc->migrate_pfn = cc->fast_start_pfn;
 	cc->fast_start_pfn = ULONG_MAX;
+
+	return cc->migrate_pfn;
 }
 
 /*
@@ -1361,7 +1363,7 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
 	 * that had free pages as the basis for starting a linear scan.
 	 */
 	if (pfn == cc->migrate_pfn)
-		reinit_migrate_pfn(cc);
+		pfn = reinit_migrate_pfn(cc);
 
 	return pfn;
 }