Re: [PATCH v3] mm/page_alloc: fix freeing of MIGRATE_RESERVE migratetype pages

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On 03/06/2014 06:35 PM, Bartlomiej Zolnierkiewicz wrote:
Pages allocated from MIGRATE_RESERVE migratetype pageblocks
are not freed back to MIGRATE_RESERVE migratetype free
lists in free_pcppages_bulk()->__free_one_page() if we got
to free_pcppages_bulk() through drain_[zone_]pages().
The freeing through free_hot_cold_page() is okay because
freepage migratetype is set to pageblock migratetype before
calling free_pcppages_bulk().

I think this is somewhat misleading and got me confused for a while. It's not about the call path of free_pcppages_bulk(), but about the fact that rmqueue_bulk() has been called at some point to fill up the pcp lists, and had to resort to __rmqueue_fallback(). So, going through free_hot_cold_page() might give you correct migratetype for the last page freed, but the pcp lists may still contain misplaced pages from earlier rmqueue_bulk().

If pages of MIGRATE_RESERVE
migratetype end up on the free lists of other migratetype
whole Reserved pageblock may be later changed to the other
migratetype in __rmqueue_fallback() and it will be never
changed back to be a Reserved pageblock.  Fix the issue by
moving freepage migratetype setting from rmqueue_bulk() to
__rmqueue[_fallback]() and preserving freepage migratetype
as an original pageblock migratetype for MIGRATE_RESERVE
migratetype pages.

Actually wouldn't the easiest solution to this particular problem to check current pageblock migratetype in try_to_steal_freepages() and disallow changing it. However I agree that preventing the misplaced page in the first place would be even better.

The problem was introduced in v2.6.31 by commit ed0ae21
("page allocator: do not call get_pageblock_migratetype()
more than necessary").

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>
Reported-by: Yong-Taek Lee <ytk.lee@xxxxxxxxxxx>
Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
---
v2:
- updated patch description, there is no __zone_pcp_update()
   in newer kernels
v3:
- set freepage migratetype in __rmqueue[_fallback]()
   instead of rmqueue_bulk() (per Mel's request)

  mm/page_alloc.c |   27 ++++++++++++++++++---------
  1 file changed, 18 insertions(+), 9 deletions(-)

Index: b/mm/page_alloc.c
===================================================================
--- a/mm/page_alloc.c	2014-03-06 18:10:21.884422983 +0100
+++ b/mm/page_alloc.c	2014-03-06 18:10:27.016422895 +0100
@@ -1094,7 +1094,7 @@ __rmqueue_fallback(struct zone *zone, in
  	struct free_area *area;
  	int current_order;
  	struct page *page;
-	int migratetype, new_type, i;
+	int migratetype, new_type, mt = start_migratetype, i;

A better naming would help, "mt" and "migratetype" are the same thing and it gets too confusing.


  	/* Find the largest possible block of pages in the other list */
  	for (current_order = MAX_ORDER-1; current_order >= order;
@@ -1125,6 +1125,14 @@ __rmqueue_fallback(struct zone *zone, in
  			expand(zone, page, order, current_order, area,
  			       new_type);

+			if (IS_ENABLED(CONFIG_CMA)) {
+				mt = get_pageblock_migratetype(page);
+				if (!is_migrate_cma(mt) &&
+				    !is_migrate_isolate(mt))
+					mt = start_migratetype;
+			}
+			set_freepage_migratetype(page, mt);
+
  			trace_mm_page_alloc_extfrag(page, order, current_order,
  				start_migratetype, migratetype, new_type);

@@ -1147,7 +1155,9 @@ static struct page *__rmqueue(struct zon
  retry_reserve:
  	page = __rmqueue_smallest(zone, order, migratetype);

-	if (unlikely(!page) && migratetype != MIGRATE_RESERVE) {
+	if (likely(page)) {
+		set_freepage_migratetype(page, migratetype);

Are you sure that here the checking of of CMA and ISOLATE is not needed? Did the original rmqueue_bulk() have this checking only for the __rmqueue_fallback() case? Why wouldn't the check already be only in __rmqueue_fallback() then?

+	} else if (migratetype != MIGRATE_RESERVE) {
  		page = __rmqueue_fallback(zone, order, migratetype);

  		/*
@@ -1174,7 +1184,7 @@ static int rmqueue_bulk(struct zone *zon
  			unsigned long count, struct list_head *list,
  			int migratetype, int cold)
  {
-	int mt = migratetype, i;
+	int i;

  	spin_lock(&zone->lock);
  	for (i = 0; i < count; ++i) {
@@ -1195,16 +1205,15 @@ static int rmqueue_bulk(struct zone *zon
  			list_add(&page->lru, list);
  		else
  			list_add_tail(&page->lru, list);
+		list = &page->lru;
  		if (IS_ENABLED(CONFIG_CMA)) {
-			mt = get_pageblock_migratetype(page);
+			int mt = get_pageblock_migratetype(page);
  			if (!is_migrate_cma(mt) && !is_migrate_isolate(mt))
  				mt = migratetype;
+			if (is_migrate_cma(mt))
+				__mod_zone_page_state(zone, NR_FREE_CMA_PAGES,
+						      -(1 << order));
  		}
-		set_freepage_migratetype(page, mt);
-		list = &page->lru;
-		if (is_migrate_cma(mt))
-			__mod_zone_page_state(zone, NR_FREE_CMA_PAGES,
-					      -(1 << order));
  	}
  	__mod_zone_page_state(zone, NR_FREE_PAGES, -(i << order));
  	spin_unlock(&zone->lock);

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