[PATCH] mm, swap: Make CONFIG_THP_SWAP depends on CONFIG_SWAP

[Huang, Ying]

From: Huang Ying <ying.huang@xxxxxxxxx>

CONFIG_THP_SWAP should depend on CONFIG_SWAP, because it's
unreasonable to optimize swapping for THP (Transparent Huge Page)
without basic swapping support.

In original code, when CONFIG_SWAP=n and CONFIG_THP_SWAP=y,
split_swap_cluster() will not be built because it is in swapfile.c,
but it will be called in huge_memory.c.  This doesn't trigger a build
error in practice because the call site is enclosed by
PageSwapCache(), which is defined to be constant 0 when CONFIG_SWAP=n.
But this is fragile and should be fixed.

The comments are fixed too to reflect the latest progress.

Fixes: 38d8b4e6bdc8 ("mm, THP, swap: delay splitting THP during swap out")
Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Reviewed-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Shaohua Li <shli@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
---
 mm/Kconfig | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index b78e7cd4e9fe..97114c94239c 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -419,10 +419,11 @@ config ARCH_WANTS_THP_SWAP
 
 config THP_SWAP
 	def_bool y
-	depends on TRANSPARENT_HUGEPAGE && ARCH_WANTS_THP_SWAP
+	depends on TRANSPARENT_HUGEPAGE && ARCH_WANTS_THP_SWAP && SWAP
 	help
 	  Swap transparent huge pages in one piece, without splitting.
-	  XXX: For now this only does clustered swap space allocation.
+	  XXX: For now, swap cluster backing transparent huge page
+	  will be split after swapout.
 
 	  For selection by architectures with reasonable THP sizes.
 
-- 
2.16.4