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Subject[PATCH] mm, compaction: Use free lists to quickly locate a migration source -fix
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@techsingularity.net>
---
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;
}
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