Re: [RFC PATCH 2/5] mm/page_alloc: use smallest fallback page first in movable allocation

From: Vlastimil Babka
Date: Thu Oct 13 2016 - 05:12:22 EST

On 10/13/2016 10:08 AM, js1304@xxxxxxxxx wrote:
From: Joonsoo Kim <>

When we try to find freepage in fallback buddy list, we always serach
the largest one. This would help for fragmentation if we process
unmovable/reclaimable allocation request because it could cause permanent
fragmentation on movable pageblock and spread out such allocations would
cause more fragmentation. But, movable allocation request is
rather different. It would be simply freed or migrated so it doesn't
contribute to fragmentation on the other pageblock. In this case, it would
be better not to break the precious highest order freepage so we need to
search the smallest freepage first.

I've also pondered this, but then found a lower hanging fruit that should be hopefully clear win and mitigate most cases of breaking high-order pages unnecessarily:

So I would try that first, and then test your patch on top? In your patch there's a risk that we make it harder for unmovable/reclaimable pageblocks to become movable again (we start with the smallest page which means there's lower chance that move_freepages_block() will convert more than half of the block). And Johannes's report seems to be about a regression in exactly this aspect of the heuristics.

Signed-off-by: Joonsoo Kim <>
mm/page_alloc.c | 26 +++++++++++++++++++++-----
1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c4f7d05..70427bf 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2121,15 +2121,31 @@ static void unreserve_highatomic_pageblock(const struct alloc_context *ac)
int fallback_mt;
bool can_steal;

- /* Find the largest possible block of pages in the other list */
- for (current_order = MAX_ORDER-1;
- current_order >= order && current_order <= MAX_ORDER-1;
- --current_order) {
+ if (start_migratetype == MIGRATE_MOVABLE)
+ current_order = order;
+ else
+ current_order = MAX_ORDER - 1;
+ /*
+ * Find the appropriate block of pages in the other list.
+ * it would be better to find largest pageblock since it could cause
+ * fragmentation. However, in case of MIGRATE_MOVABLE, there is no
+ * risk about fragmentation so it would be better to use smallest one.
+ */
+ while (current_order >= order && current_order <= MAX_ORDER - 1) {
area = &(zone->free_area[current_order]);
fallback_mt = find_suitable_fallback(area, current_order,
start_migratetype, false, &can_steal);
- if (fallback_mt == -1)
+ if (fallback_mt == -1) {
+ if (start_migratetype == MIGRATE_MOVABLE)
+ current_order++;
+ else
+ current_order--;
+ }

page = list_first_entry(&area->free_list[fallback_mt],
struct page, lru);