[PATCH] mm, page_alloc: prevent infinite loop in buffered_rmqueue()

[Vlastimil Babka]

In DEBUG_VM kernel, we can hit infinite loop for order == 0 in
buffered_rmqueue() when check_new_pcp() returns 1, because the bad page is
never removed from the pcp list. Fix this by removing the page before retrying.
Also we don't need to check if page is non-NULL, because we simply grab it from
the list which was just tested for being non-empty.

Fixes: http://www.ozlabs.org/~akpm/mmotm/broken-out/mm-page_alloc-defer-debugging-checks-of-freed-pages-until-a-pcp-drain.patch
Reported-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/page_alloc.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 8c81e2e7b172..d5b93e5dd697 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2641,11 +2641,12 @@ struct page *buffered_rmqueue(struct zone *preferred_zone,
 				page = list_last_entry(list, struct page, lru);
 			else
 				page = list_first_entry(list, struct page, lru);
-		} while (page && check_new_pcp(page));
 
-		__dec_zone_state(zone, NR_ALLOC_BATCH);
-		list_del(&page->lru);
-		pcp->count--;
+			__dec_zone_state(zone, NR_ALLOC_BATCH);
+			list_del(&page->lru);
+			pcp->count--;
+
+		} while (check_new_pcp(page));
 	} else {
 		/*
 		 * We most definitely don't want callers attempting to
-- 
2.8.2