[PATCH 1/2] mm/debug-pagealloc: correct freepage accounting and order resetting

[Joonsoo Kim]

One thing, I did in this patch, is fixing freepage accounting.
If we clear guard page and link it onto isolate buddy list, we should
not increase freepage count. This patch adds conditional branch to
skip counting in this case. Without this patch, this overcounting
happens frequently if guard order is set and CMA is used.

Another thing fixed in this patch is the target to reset order. In
__free_one_page(), we check the buddy page whether it is a guard page or
not. And, if so, we should clear guard attribute on the buddy page and
reset order of it to 0. But, current code resets original page's order
rather than buddy one's. Maybe, this doesn't have any problem, because
whole merged page's order will be re-assigned soon. But, it is better
to correct code.

Changes from v2:
Rename subject from
"mm/page_alloc: correct to clear guard attribute in DEBUG_PAGEALLOC"
to
"mm/debug-pagealloc: correct freepage accounting and order resetting".
Separate fix and clean-up part.

Cc: <stable@xxxxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
---
 mm/page_alloc.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e78e3c8..d673f64 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -583,9 +583,11 @@ static inline void __free_one_page(struct page *page,
 		 */
 		if (page_is_guard(buddy)) {
 			clear_page_guard_flag(buddy);
-			set_page_private(page, 0);
-			__mod_zone_freepage_state(zone, 1 << order,
-						  migratetype);
+			set_page_private(buddy, 0);
+			if (!is_migrate_isolate(migratetype)) {
+				__mod_zone_freepage_state(zone, 1 << order,
+							  migratetype);
+			}
 		} else {
 			list_del(&buddy->lru);
 			zone->free_area[order].nr_free--;
-- 
1.7.9.5

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