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Subject[PATCH v4 1/4] mm/page_alloc: fix incorrect isolation behavior by rechecking migratetype
There are two paths to reach core free function of buddy allocator,
__free_one_page(), one is free_one_page()->__free_one_page() and the
other is free_hot_cold_page()->free_pcppages_bulk()->__free_one_page().
Each paths has race condition causing serious problems. At first, this
patch is focused on first type of freepath. And then, following patch
will solve the problem in second type of freepath.

In the first type of freepath, we got migratetype of freeing page without
holding the zone lock, so it could be racy. There are two cases of this

1. pages are added to isolate buddy list after restoring orignal


get migratetype => return MIGRATE_ISOLATE
call free_one_page() with MIGRATE_ISOLATE

grab the zone lock
unisolate pageblock
release the zone lock

grab the zone lock
call __free_one_page() with MIGRATE_ISOLATE
freepage go into isolate buddy list,
although pageblock is already unisolated

This may cause two problems. One is that we can't use this page anymore
until next isolation attempt of this pageblock, because freepage is on
isolate buddy list. The other is that freepage accouting could be wrong
due to merging between different buddy list. Freepages on isolate buddy
list aren't counted as freepage, but ones on normal buddy list are counted
as freepage. If merge happens, buddy freepage on normal buddy list is
inevitably moved to isolate buddy list without any consideration of
freepage accouting so it could be incorrect.

2. pages are added to normal buddy list while pageblock is isolated.
It is similar with above case.

This also may cause two problems. One is that we can't keep these
freepages from being allocated. Although this pageblock is isolated,
freepage would be added to normal buddy list so that it could be
allocated without any restriction. And the other problem is same as
case 1, that it, incorrect freepage accouting.

This race condition would be prevented by checking migratetype again
with holding the zone lock. Because it is somewhat heavy operation
and it isn't needed in common case, we want to avoid rechecking as much
as possible. So this patch introduce new variable, nr_isolate_pageblock
in struct zone to check if there is isolated pageblock.
With this, we can avoid to re-check migratetype in common case and do
it only if there is isolated pageblock or migratetype is MIGRATE_ISOLATE.
This solve above mentioned problems.

Changes from v3:
Add one more check in free_one_page() that checks whether migratetype is
MIGRATE_ISOLATE or not. Without this, abovementioned case 1 could happens.

Cc: <>
Signed-off-by: Joonsoo Kim <>
include/linux/mmzone.h | 9 +++++++++
include/linux/page-isolation.h | 8 ++++++++
mm/page_alloc.c | 11 +++++++++--
mm/page_isolation.c | 2 ++
4 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 4593567..3d090af 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -431,6 +431,15 @@ struct zone {
int nr_migrate_reserve_block;

+ /*
+ * Number of isolated pageblock. It is used to solve incorrect
+ * freepage counting problem due to racy retrieving migratetype
+ * of pageblock. Protected by zone->lock.
+ */
+ unsigned long nr_isolate_pageblock;
/* see spanned/present_pages for more description */
seqlock_t span_seqlock;
diff --git a/include/linux/page-isolation.h b/include/linux/page-isolation.h
index 3fff8e7..2dc1e16 100644
--- a/include/linux/page-isolation.h
+++ b/include/linux/page-isolation.h
@@ -2,6 +2,10 @@

+static inline bool has_isolate_pageblock(struct zone *zone)
+ return zone->nr_isolate_pageblock;
static inline bool is_migrate_isolate_page(struct page *page)
return get_pageblock_migratetype(page) == MIGRATE_ISOLATE;
@@ -11,6 +15,10 @@ static inline bool is_migrate_isolate(int migratetype)
return migratetype == MIGRATE_ISOLATE;
+static inline bool has_isolate_pageblock(struct zone *zone)
+ return false;
static inline bool is_migrate_isolate_page(struct page *page)
return false;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 70027da..4a5d8e5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -749,9 +749,16 @@ static void free_one_page(struct zone *zone,
if (nr_scanned)
__mod_zone_page_state(zone, NR_PAGES_SCANNED, -nr_scanned);

+ if (unlikely(has_isolate_pageblock(zone) ||
+ is_migrate_isolate(migratetype))) {
+ migratetype = get_pfnblock_migratetype(page, pfn);
+ if (is_migrate_isolate(migratetype))
+ goto skip_counting;
+ }
+ __mod_zone_freepage_state(zone, 1 << order, migratetype);
__free_one_page(page, pfn, zone, order, migratetype);
- if (unlikely(!is_migrate_isolate(migratetype)))
- __mod_zone_freepage_state(zone, 1 << order, migratetype);

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index d1473b2..1fa4a4d 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -60,6 +60,7 @@ out:
int migratetype = get_pageblock_migratetype(page);

set_pageblock_migratetype(page, MIGRATE_ISOLATE);
+ zone->nr_isolate_pageblock++;
nr_pages = move_freepages_block(zone, page, MIGRATE_ISOLATE);

__mod_zone_freepage_state(zone, -nr_pages, migratetype);
@@ -83,6 +84,7 @@ void unset_migratetype_isolate(struct page *page, unsigned migratetype)
nr_pages = move_freepages_block(zone, page, migratetype);
__mod_zone_freepage_state(zone, nr_pages, migratetype);
set_pageblock_migratetype(page, migratetype);
+ zone->nr_isolate_pageblock--;
spin_unlock_irqrestore(&zone->lock, flags);

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