[PATCH 3/3] mm: Clear page->private when splitting or migrating a page

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

In our efforts to remove uses of PG_private, we have found folios with
the private flag clear and folio->private not-NULL.  That is the root
cause behind 642d51fb0775 ("ceph: check folio PG_private bit instead
of folio->private").  It can also affect a few other filesystems that
haven't yet reported a problem.

compaction_alloc() can return a page with uninitialised page->private,
and rather than checking all the callers of migrate_pages(), just zero
page->private after calling get_new_page().  Similarly, the tail pages
from split_huge_page() may also have an uninitialised page->private.

Reported-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 mm/huge_memory.c | 1 +
 mm/migrate.c     | 1 +
 2 files changed, 2 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f7248002dad9..9b31a50217b5 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2377,6 +2377,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
 			page_tail);
 	page_tail->mapping = head->mapping;
 	page_tail->index = head->index + tail;
+	page_tail->private = NULL;
 
 	/* Page flags must be visible before we make the page non-compound. */
 	smp_wmb();
diff --git a/mm/migrate.c b/mm/migrate.c
index e51588e95f57..6c1ea61f39d8 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1106,6 +1106,7 @@ static int unmap_and_move(new_page_t get_new_page,
 	if (!newpage)
 		return -ENOMEM;
 
+	newpage->private = 0;
 	rc = __unmap_and_move(page, newpage, force, mode);
 	if (rc == MIGRATEPAGE_SUCCESS)
 		set_page_owner_migrate_reason(newpage, reason);
-- 
2.35.1