[PATCH v2 2/6] mm/balloon_compaction: keep ballooned pages away from normal migration path

[Konstantin Khlebnikov]

From: Konstantin Khlebnikov <k.khlebnikov@xxxxxxxxxxx>

Proper testing shows yet another problem in balloon migration: it works only
once for each page. balloon_page_movable() check page flags and page_count.
In __unmap_and_move page is locked, reference counter is elevated, so
balloon_page_movable() _always_ fails here. As result in __unmap_and_move()
migration goes to the normal migration path.

Balloon ->migratepage() is so special, it returns MIGRATEPAGE_BALLOON_SUCCESS
instead of MIGRATEPAGE_SUCCESS. After that in move_to_new_page() successfully
migrated page got NULL into its mapping pointer and loses connectivity with
balloon and ability for further migration.

It's safe to use __is_movable_balloon_page here: page is isolated and pinned.

Signed-off-by: Konstantin Khlebnikov <k.khlebnikov@xxxxxxxxxxx>
Cc: stable <stable@xxxxxxxxxxxxxxx> # v3.8
---
 include/linux/balloon_compaction.h |    5 +++++
 mm/migrate.c                       |    2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
index 53d482e..284fc1d 100644
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h
@@ -258,6 +258,11 @@ static inline void balloon_page_delete(struct page *page)
 	list_del(&page->lru);
 }
 
+static inline bool __is_movable_balloon_page(struct page *page)
+{
+	return false;
+}
+
 static inline bool balloon_page_movable(struct page *page)
 {
 	return false;
diff --git a/mm/migrate.c b/mm/migrate.c
index 905b1aa..57c94f9 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -873,7 +873,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 		}
 	}
 
-	if (unlikely(balloon_page_movable(page))) {
+	if (unlikely(__is_movable_balloon_page(page))) {
 		/*
 		 * A ballooned page does not need any special attention from
 		 * physical to virtual reverse mapping procedures.

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