[PATCH v5 08/19] mm, swap: require zswap for xswap devices

[Baoquan He <baoquan.he@linux.dev>]

From: Baoquan He <hebaoquan@kylinos.cn>

xswap pages live only in zswap.  Without zswap, swapout cannot
free the page, but still takes a swap entry.  So the device consumes
swap entries without freeing memory.  Fail to create a device when
zswap is unavailable.

Acked-by: Chris Li <chrisl@kernel.org>
Signed-off-by: Baoquan He <hebaoquan@kylinos.cn>
---
 mm/swapfile.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 3f2b863f7dd7..35c8aac1e25d 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -4268,6 +4268,10 @@ static int xswap_create(int prio)
 	    (prio < 0 || prio > SWAP_FLAG_PRIO_MASK))
 		return -EINVAL;
 
+	/* xswap has no backing store, it relies on zswap. */
+	if (!zswap_is_enabled())
+		return -EOPNOTSUPP;
+
 	si = alloc_swap_info();
 	if (IS_ERR(si))
 		return PTR_ERR(si);
-- 
2.54.0