mm/z3fold.c: reinitialize zhdr structs after migration

z3fold_page_migration() calls memcpy(new_zhdr, zhdr, PAGE_SIZE).
However, zhdr contains fields that can't be directly coppied over (ex:
list_head, a circular linked list).  We only need to initialize the
linked lists in new_zhdr, as z3fold_isolate_page() already ensures that
these lists are empty

Additionally it is possible that zhdr->work has been placed in a
workqueue.  In this case we shouldn't migrate the page, as zhdr->work
references zhdr as opposed to new_zhdr.

Link: http://lkml.kernel.org/r/20190716000520.230595-1-henryburns@google.com
Fixes: 1f862989 ("mm/z3fold.c: support page migration")
Signed-off-by: Henry Burns <henryburns@google.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Vitaly Vul <vitaly.vul@sony.com>
Cc: Vitaly Wool <vitalywool@gmail.com>
Cc: Jonathan Adams <jwadams@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/z3fold.c

@@ -1357,12 +1357,22 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
 		z3fold_page_unlock(zhdr);
 		return -EBUSY;
 	}
+	if (work_pending(&zhdr->work)) {
+		z3fold_page_unlock(zhdr);
+		return -EAGAIN;
+	}
 	new_zhdr = page_address(newpage);
 	memcpy(new_zhdr, zhdr, PAGE_SIZE);
 	newpage->private = page->private;
 	page->private = 0;
 	z3fold_page_unlock(zhdr);
 	spin_lock_init(&new_zhdr->page_lock);
+	INIT_WORK(&new_zhdr->work, compact_page_work);
+	/*
+	 * z3fold_page_isolate() ensures that new_zhdr->buddy is empty,
+	 * so we only have to reinitialize it.
+	 */
+	INIT_LIST_HEAD(&new_zhdr->buddy);
 	new_mapping = page_mapping(page);
 	__ClearPageMovable(page);
 	ClearPagePrivate(page);