mm: zswap: fix zswap_never_enabled() for CONFIG_ZSWAP==N

If CONFIG_ZSWAP is set to N, it means zswap cannot be enabled. 
zswap_never_enabled() should return true.

The only effect of this issue is that with Barry's latest large folio
swapin patches for zram ("mm: support mTHP swap-in for zRAM-like
swapfile"), we will always fallback to order-0 swapin, even mistakenly
when !CONFIG_ZSWAP.

Basically this bug makes Barry's in progress patches not work at all.

The API was created to inform the mm core that zswap has never been
enabled, allowing the mm core to perform mTHP swap-in.  This is a
transitional solution until zswap supports mTHP.  If zswap has been
enabled, performing mTHP swap-in will result in corrupted data.  You
may find the answer in the mTHP swap-in series:

https://lore.kernel.org/linux-mm/CAJD7tkZ4FQr6HZpduOdvmqgg_-whuZYE-Bz5O2t6yzw6Yg+v1A@mail.gmail.com/

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 0300e17d67c3 ("mm: zswap: add zswap_never_enabled()")
Signed-off-by: Barry Song <[email protected]>
Reviewed-by: Chengming Zhou <[email protected]>
Acked-by: Yosry Ahmed <[email protected]>
Acked-by: Chris Li <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Reviewed-by: Nhat Pham <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/linux/zswap.h b/include/linux/zswap.h
index bf83ae5e285d..6cecb4a4f68b 100644
--- a/include/linux/zswap.h
+++ b/include/linux/zswap.h
@@ -68,7 +68,7 @@ static inline bool zswap_is_enabled(void)
 
 static inline bool zswap_never_enabled(void)
 {
-	return false;
+	return true;
 }
 
 #endif