[-mm][PATCH 2/5] migration_entry_wait fix.

[KOSAKI Motohiro]

From: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>

In speculative page cache lookup protocol, page_count(page) is set to 0
while radix-tree modification is going on, truncation, migration, etc...

While page migration, a page fault to page under migration should wait
unlock_page() and migration_entry_wait() waits for the page from its
pte entry. It does get_page() -> wait_on_page_locked() -> put_page() now.

In page migration, page_freeze_refs() -> page_unfreeze_refs() is called.

Here, page_unfreeze_refs() expects page_count(page) == 0 and panics
if page_count(page) != 0. To avoid this, we shouldn't touch page_count()
if it is zero. This patch uses page_cache_get_speculative() to avoid
the panic.

Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Acked-by: Nick Piggin <npiggin@xxxxxxx>

---
 mm/migrate.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

Index: b/mm/migrate.c
===================================================================
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -243,7 +243,15 @@ void migration_entry_wait(struct mm_stru
 
 	page = migration_entry_to_page(entry);
 
-	get_page(page);
+	/*
+	 * Once radix-tree replacement of page migration started, page_count
+	 * *must* be zero. And, we don't want to call wait_on_page_locked()
+	 * against a page without get_page().
+	 * So, we use get_page_unless_zero(), here. Even failed, page fault
+	 * will occur again.
+	 */
+	if (!get_page_unless_zero(page))
+		goto out;
 	pte_unmap_unlock(ptep, ptl);
 	wait_on_page_locked(page);
 	put_page(page);


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