mm/z3fold.c: lock z3fold page before __SetPageMovable()

Following zsmalloc.c's example we call trylock_page() and unlock_page().
Also make z3fold_page_migrate() assert that newpage is passed in locked,
as per the documentation.

[[email protected]: fix trylock_page return value test, per Shakeel]
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Henry Burns <[email protected]>
Suggested-by: Vitaly Wool <[email protected]>
Acked-by: Vitaly Wool <[email protected]>
Acked-by: David Rientjes <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Cc: Vitaly Vul <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Xidong Wang <[email protected]>
Cc: Jonathan Adams <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/z3fold.c

@@ -924,7 +924,16 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
 		set_bit(PAGE_HEADLESS, &page->private);
 		goto headless;
 	}
-	__SetPageMovable(page, pool->inode->i_mapping);
+	if (can_sleep) {
+		lock_page(page);
+		__SetPageMovable(page, pool->inode->i_mapping);
+		unlock_page(page);
+	} else {
+		if (trylock_page(page)) {
+			__SetPageMovable(page, pool->inode->i_mapping);
+			unlock_page(page);
+		}
+	}
 	z3fold_page_lock(zhdr);
 
 found:
@@ -1331,6 +1340,7 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
 
 	VM_BUG_ON_PAGE(!PageMovable(page), page);
 	VM_BUG_ON_PAGE(!PageIsolated(page), page);
+	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
 
 	zhdr = page_address(page);
 	pool = zhdr_to_pool(zhdr);