[PATCH v4 10/14] mm, swap: require zswap for xswap devices

[Baoquan He]

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.

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

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 0c8b716ec5ba..8c8e99ea403e 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -4504,6 +4504,10 @@ 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