mm, swap: make CONFIG_THP_SWAP depend on CONFIG_SWAP

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.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 38d8b4e ("mm, THP, swap: delay splitting THP during swap out")
Signed-off-by: "Huang, Ying" <[email protected]>
Reviewed-by: Dan Williams <[email protected]>
Reviewed-by: Naoya Horiguchi <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Shaohua Li <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: Daniel Jordan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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.