[PATCH v2] mm/z3fold.c: Lock z3fold page before __SetPageMovable()

[Henry Burns <henryburns@google.com>]

__SetPageMovable() expects it's page to be locked, but z3fold.c doesn't
lock the page. Following zsmalloc.c's example we call trylock_page() and
unlock_page(). Also makes z3fold_page_migrate() assert that newpage is
passed in locked, as documentation.

Signed-off-by: Henry Burns <henryburns@google.com>
Suggested-by: Vitaly Wool <vitalywool@gmail.com>
---
 Changelog since v1:
 - Added an if statement around WARN_ON(trylock_page(page)) to avoid
   unlocking a page locked by a someone else.

 mm/z3fold.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index e174d1549734..6341435b9610 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -918,7 +918,10 @@ 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 (!WARN_ON(!trylock_page(page))) {
+		__SetPageMovable(page, pool->inode->i_mapping);
+		unlock_page(page);
+	}
 	z3fold_page_lock(zhdr);
 
 found:
@@ -1325,6 +1328,7 @@ 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);
-- 
2.22.0.410.gd8fdbe21b5-goog