[PATCH v3] mm/page_isolation: guard against possible putback unisolated page

[Miaohe Lin]

Isolating a free page in an isolated pageblock is expected to always work
as watermarks don't apply here. But if __isolate_free_page() failed, due
to condition changes, the page will be left on the free list. And the page
will be put back to free list again via __putback_isolated_page(). This may
trigger VM_BUG_ON_PAGE() on page->flags checking in __free_one_page() if
PageReported is set. Or we will corrupt the free list because list_add()
will be called for pages already on another list. Add a VM_WARN_ON() to
complain about this change.

Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
---
v1->v2:
  Add VM_WARN_ON to catch unexpected isolating failure.
v2->v3:
  Change the patch description+subject and remove Fixes.
---
 mm/page_isolation.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index f93cc63d8fa1..f67c4c70f17f 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -94,8 +94,13 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
 			buddy = page + (buddy_pfn - pfn);
 
 			if (!is_migrate_isolate_page(buddy)) {
-				__isolate_free_page(page, order);
-				isolated_page = true;
+				isolated_page = !!__isolate_free_page(page, order);
+				/*
+				 * Isolating a free page in an isolated pageblock
+				 * is expected to always work as watermarks don't
+				 * apply here.
+				 */
+				VM_WARN_ON(!isolated_page);
 			}
 		}
 	}
-- 
2.23.0