slub: document setting min order with debug_guardpage_minorder > 0

Acked-by: David Rientjes <rientjes@google.com>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/ABI/testing/sysfs-kernel-slab

@@ -346,6 +346,10 @@ Description:
 		number of objects per slab.  If a slab cannot be allocated
 		because of fragmentation, SLUB will retry with the minimum order
 		possible depending on its characteristics.
+		When debug_guardpage_minorder=N (N > 0) parameter is specified
+		(see Documentation/kernel-parameters.txt), the minimum possible
+		order is used and this sysfs entry can not be used to change
+		the order at run time.
 
 What:		/sys/kernel/slab/cache/order_fallback
 Date:		April 2008

Documentation/vm/slub.txt

@@ -131,7 +131,10 @@ slub_min_objects.
 slub_max_order specified the order at which slub_min_objects should no
 longer be checked. This is useful to avoid SLUB trying to generate
 super large order pages to fit slub_min_objects of a slab cache with
-large object sizes into one high order page.
+large object sizes into one high order page. Setting command line
+parameter debug_guardpage_minorder=N (N > 0), forces setting
+slub_max_order to 0, what cause minimum possible order of slabs
+allocation.
 
 SLUB Debug output
 -----------------