mm: Update shuffle documentation to match its current state

Commit 8391953 ("mm/shuffle: remove dynamic reconfiguration")
removed the dynamic reconfiguration capabilities from the shuffle page
allocator. This means that, now, we don't have any perspective of an
"autodetection of memory-side-cache" that triggers the enablement of the
shuffle page allocator.

Therefore, let the documentation reflect that the only way to enable
the shuffle page allocator is by setting `page_alloc.shuffle=1`.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240422142007.1062231-1-mcanal@igalia.com

Author: mairacanal

Documentation/admin-guide/kernel-parameters.txt

@@ -4169,13 +4169,11 @@
 
 	page_alloc.shuffle=
 			[KNL] Boolean flag to control whether the page allocator
-			should randomize its free lists. The randomization may
-			be automatically enabled if the kernel detects it is
-			running on a platform with a direct-mapped memory-side
-			cache, and this parameter can be used to
-			override/disable that behavior. The state of the flag
-			can be read from sysfs at:
+			should randomize its free lists. This parameter can be
+			used to enable/disable page randomization. The state of
+			the flag can be read from sysfs at:
 			/sys/module/page_alloc/parameters/shuffle.
+			This parameter is only available if CONFIG_SHUFFLE_PAGE_ALLOCATOR=y.
 
 	page_owner=	[KNL,EARLY] Boot-time page_owner enabling option.
 			Storage of the information about who allocated

mm/Kconfig

@@ -333,10 +333,9 @@ config SHUFFLE_PAGE_ALLOCATOR
 
 	  While the randomization improves cache utilization it may
 	  negatively impact workloads on platforms without a cache. For
-	  this reason, by default, the randomization is enabled only
-	  after runtime detection of a direct-mapped memory-side-cache.
-	  Otherwise, the randomization may be force enabled with the
-	  'page_alloc.shuffle' kernel command line parameter.
+	  this reason, by default, the randomization is not enabled even
+	  if SHUFFLE_PAGE_ALLOCATOR=y. The randomization may be force enabled
+	  with the 'page_alloc.shuffle' kernel command line parameter.
 
 	  Say Y if unsure.