[RFC v3 15/15] mm, swap: require zswap for xswap devices

xswap has no backing storage: swapped-out pages live only in zswap.
Without zswap, swapout always bounces back, so the device would
consume swap entry space without ever freeing memory.  Refuse to
create a device when zswap is unavailable.

Runtime disabling of zswap after creation is safe: existing entries
stay loadable (zswap_load() gates on zswap_never_enabled(), not the
runtime zswap_enabled flag) and new swapouts merely bounce back to
memory without freeing it.

Signed-off-by: Baoquan He <[email protected]>
---
 mm/swapfile.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 61c307b6a432..911bf3b8c863 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -4340,6 +4340,10 @@ static int xswap_create(int percent)
 	if (percent < 1 || percent > 100)
 		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