mm: clarify CONFIG_PAGE_POISONING and usage

The Kconfig text for CONFIG_PAGE_POISONING doesn't mention that it has to
be enabled explicitly.  This updates the documentation for that and adds a
note about CONFIG_PAGE_POISONING to the "page_poison" command line docs.
While here, change description of CONFIG_PAGE_POISONING_ZERO too, as it's
not "random" data, but rather the fixed debugging value that would be used
when not zeroing.  Additionally removes a stray "bool" in the Kconfig.

Link: http://lkml.kernel.org/r/20180725223832.GA43733@beast
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Laura Abbott <labbott@redhat.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -3041,8 +3041,9 @@
 			on: enable the feature
 
 	page_poison=	[KNL] Boot-time parameter changing the state of
-			poisoning on the buddy allocator.
-			off: turn off poisoning
+			poisoning on the buddy allocator, available with
+			CONFIG_PAGE_POISONING=y.
+			off: turn off poisoning (default)
 			on: turn on poisoning
 
 	panic=		[KNL] Kernel behaviour on panic: delay <timeout>

mm/Kconfig.debug

@@ -46,7 +46,8 @@ config PAGE_POISONING
 	  Fill the pages with poison patterns after free_pages() and verify
 	  the patterns before alloc_pages. The filling of the memory helps
 	  reduce the risk of information leaks from freed data. This does
-	  have a potential performance impact.
+	  have a potential performance impact if enabled with the
+	  "page_poison=1" kernel boot option.
 
 	  Note that "poison" here is not the same thing as the "HWPoison"
 	  for CONFIG_MEMORY_FAILURE. This is software poisoning only.
@@ -65,7 +66,7 @@ config PAGE_POISONING_NO_SANITY
 	   say N.
 
 config PAGE_POISONING_ZERO
-	bool "Use zero for poisoning instead of random data"
+	bool "Use zero for poisoning instead of debugging value"
 	depends on PAGE_POISONING
 	---help---
 	   Instead of using the existing poison value, fill the pages with
@@ -75,7 +76,6 @@ config PAGE_POISONING_ZERO
 	   allocation.
 
 	   If unsure, say N
-	bool
 
 config DEBUG_PAGE_REF
 	bool "Enable tracepoint to track down page reference manipulation"