mm/slab_common: provide "slab_merge" option for !IS_ENABLED(CONFIG_SL…

…AB_MERGE_DEFAULT) builds

This is a minor addition to the allocator setup options to provide
a simple way to on demand enable back cache merging for builds
that by default run with CONFIG_SLAB_MERGE_DEFAULT not set.

Signed-off-by: Rafael Aquini <aquini@redhat.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -4770,6 +4770,10 @@
 
 	slram=		[HW,MTD]
 
+	slab_merge	[MM]
+			Enable merging of slabs with similar size when the
+			kernel is built without CONFIG_SLAB_MERGE_DEFAULT.
+
 	slab_nomerge	[MM]
 			Disable merging of slabs with similar size. May be
 			necessary if there is some reason to distinguish
@@ -4825,6 +4829,9 @@
 			lower than slub_max_order.
 			For more information see Documentation/vm/slub.rst.
 
+	slub_merge	[MM, SLUB]
+			Same with slab_merge.
+
 	slub_nomerge	[MM, SLUB]
 			Same with slab_nomerge. This is supported for legacy.
 			See slab_nomerge for more information.

mm/slab_common.c

@@ -69,11 +69,19 @@ static int __init setup_slab_nomerge(char *str)
 	return 1;
 }
 
+static int __init setup_slab_merge(char *str)
+{
+	slab_nomerge = false;
+	return 1;
+}
+
 #ifdef CONFIG_SLUB
 __setup_param("slub_nomerge", slub_nomerge, setup_slab_nomerge, 0);
+__setup_param("slub_merge", slub_merge, setup_slab_merge, 0);
 #endif
 
 __setup("slab_nomerge", setup_slab_nomerge);
+__setup("slab_merge", setup_slab_nomerge);
 
 /*
  * Determine the size of a slab object