From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
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@mxp.nes.nec.co.jp>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Nick Piggin <npiggin@suse.de>
---
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);
--
