mm: Clear page->private when splitting or migrating a page

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 642d51fb ("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>
Tested-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

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 = 0;
 
 	/* Page flags must be visible before we make the page non-compound. */
 	smp_wmb();

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);