[PATCH 2/6] mm/z3fold: avoid possible underflow in z3fold_alloc()

[Miaohe Lin <linmiaohe@huawei.com>]

It is not enough to just make sure the z3fold header is not larger than the
page size. When z3fold header is equal to PAGE_SIZE, we would underflow
when check alloc size against PAGE_SIZE - ZHDR_SIZE_ALIGNED - CHUNK_SIZE
in z3fold_alloc(). Make sure there has remaining spaces for its buddy to
fix this theoretical issue.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
This causes the below checkpatch warning:
WARNING: Comparisons should place the constant on the right side of the
test
#31: FILE: mm/z3fold.c:1812:
+       BUILD_BUG_ON(ZHDR_SIZE_ALIGNED > PAGE_SIZE - CHUNK_SIZE);

But I think the error is false positives as all members are constant.
---
 mm/z3fold.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index 04d0e493bd2e..e261e14b7753 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -1805,8 +1805,11 @@ static int __init init_z3fold(void)
 {
 	int ret;
 
-	/* Make sure the z3fold header is not larger than the page size */
-	BUILD_BUG_ON(ZHDR_SIZE_ALIGNED > PAGE_SIZE);
+	/*
+	 * Make sure the z3fold header is not larger than the page size and
+	 * there has remaining spaces for its buddy.
+	 */
+	BUILD_BUG_ON(ZHDR_SIZE_ALIGNED > PAGE_SIZE - CHUNK_SIZE);
 	ret = z3fold_mount();
 	if (ret)
 		return ret;
-- 
2.23.0