SLUB: support slub_debug on by default

Add a new configuration variable

CONFIG_SLUB_DEBUG_ON

If set then the kernel will be booted by default with slab debugging
switched on. Similar to CONFIG_SLAB_DEBUG. By default slab debugging
is available but must be enabled by specifying "slub_debug" as a
kernel parameter.

Also add support to switch off slab debugging for a kernel that was
built with CONFIG_SLUB_DEBUG_ON. This works by specifying

slub_debug=-

as a kernel parameter.

Dave Jones wanted this feature.
http://marc.info/?l=linux-kernel&m=118072189913045&w=2

[akpm@linux-foundation.org: clean up switch statement]
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/kernel-parameters.txt

@@ -1580,35 +1580,39 @@ and is between 256 and 4096 characters. It is defined in the file
 
 	slram=		[HW,MTD]
 
-	slub_debug	[MM, SLUB]
-			Enabling slub_debug allows one to determine the culprit
-			if slab objects become corrupted. Enabling slub_debug
-			creates guard zones around objects and poisons objects
-			when not in use. Also tracks the last alloc / free.
-			For more information see Documentation/vm/slub.txt.
+	slub_debug[=options[,slabs]]	[MM, SLUB]
+			Enabling slub_debug allows one to determine the
+			culprit if slab objects become corrupted. Enabling
+			slub_debug can create guard zones around objects and
+			may poison objects when not in use. Also tracks the
+			last alloc / free. For more information see
+			Documentation/vm/slub.txt.
 
 	slub_max_order= [MM, SLUB]
-			Determines the maximum allowed order for slabs. Setting
-			this too high may cause fragmentation.
-			For more information see Documentation/vm/slub.txt.
+			Determines the maximum allowed order for slabs.
+			A high setting may cause OOMs due to memory
+			fragmentation. For more information see
+			Documentation/vm/slub.txt.
 
 	slub_min_objects=	[MM, SLUB]
-			The minimum objects per slab. SLUB will increase the
-			slab order up to slub_max_order to generate a
-			sufficiently big slab to satisfy the number of objects.
-			The higher the number of objects the smaller the overhead
-			of tracking slabs.
+			The minimum number of objects per slab. SLUB will
+			increase the slab order up to slub_max_order to
+			generate a sufficiently large slab able to contain
+			the number of objects indicated. The higher the number
+			of objects the smaller the overhead of tracking slabs
+			and the less frequently locks need to be acquired.
 			For more information see Documentation/vm/slub.txt.
 
 	slub_min_order=	[MM, SLUB]
 			Determines the mininum page order for slabs. Must be
-			lower than slub_max_order
+			lower than slub_max_order.
 			For more information see Documentation/vm/slub.txt.
 
 	slub_nomerge	[MM, SLUB]
-			Disable merging of slabs of similar size. May be
+			Disable merging of slabs with similar size. May be
 			necessary if there is some reason to distinguish
-			allocs to different slabs.
+			allocs to different slabs. Debug options disable
+			merging on their own.
 			For more information see Documentation/vm/slub.txt.
 
 	smart2=		[HW]

Documentation/vm/slub.txt

@@ -41,6 +41,8 @@ Possible debug options are
 	P		Poisoning (object and padding)
 	U		User tracking (free and alloc)
 	T		Trace (please only use on single slabs)
+	-		Switch all debugging off (useful if the kernel is
+			configured with CONFIG_SLUB_DEBUG_ON)
 
 F.e. in order to boot just with sanity checks and red zoning one would specify:
 

lib/Kconfig.debug

@@ -152,6 +152,19 @@ config DEBUG_SLAB_LEAK
 	bool "Memory leak debugging"
 	depends on DEBUG_SLAB
 
+config SLUB_DEBUG_ON
+	bool "SLUB debugging on by default"
+	depends on SLUB && SLUB_DEBUG
+	default n
+	help
+	  Boot with debugging on by default. SLUB boots by default with
+	  the runtime debug capabilities switched off. Enabling this is
+	  equivalent to specifying the "slub_debug" parameter on boot.
+	  There is no support for more fine grained debug control like
+	  possible with slub_debug=xxx. SLUB debugging may be switched
+	  off in a kernel built with CONFIG_SLUB_DEBUG_ON by specifying
+	  "slub_debug=-".
+
 config DEBUG_PREEMPT
 	bool "Debug preemptible kernel"
 	depends on DEBUG_KERNEL && PREEMPT && TRACE_IRQFLAGS_SUPPORT

mm/slub.c

@@ -323,7 +323,11 @@ static inline int slab_index(void *p, struct kmem_cache *s, void *addr)
 /*
  * Debug settings:
  */
+#ifdef CONFIG_SLUB_DEBUG_ON
+static int slub_debug = DEBUG_DEFAULT_FLAGS;
+#else
 static int slub_debug;
+#endif
 
 static char *slub_debug_slabs;
 
@@ -888,28 +892,45 @@ fail:
 
 static int __init setup_slub_debug(char *str)
 {
-	if (!str || *str != '=')
 	slub_debug = DEBUG_DEFAULT_FLAGS;
-	else {
-		str++;
-		if (*str == 0 || *str == ',')
-			slub_debug = DEBUG_DEFAULT_FLAGS;
-		else
-		for( ;*str && *str != ','; str++)
-			switch (*str) {
-			case 'f' : case 'F' :
+	if (*str++ != '=' || !*str)
+		/*
+		 * No options specified. Switch on full debugging.
+		 */
+		goto out;
+
+	if (*str == ',')
+		/*
+		 * No options but restriction on slabs. This means full
+		 * debugging for slabs matching a pattern.
+		 */
+		goto check_slabs;
+
+	slub_debug = 0;
+	if (*str == '-')
+		/*
+		 * Switch off all debugging measures.
+		 */
+		goto out;
+
+	/*
+	 * Determine which debug features should be switched on
+	 */
+	for ( ;*str && *str != ','; str++) {
+		switch (tolower(*str)) {
+		case 'f':
 			slub_debug |= SLAB_DEBUG_FREE;
 			break;
-			case 'z' : case 'Z' :
+		case 'z':
 			slub_debug |= SLAB_RED_ZONE;
 			break;
-			case 'p' : case 'P' :
+		case 'p':
 			slub_debug |= SLAB_POISON;
 			break;
-			case 'u' : case 'U' :
+		case 'u':
 			slub_debug |= SLAB_STORE_USER;
 			break;
-			case 't' : case 'T' :
+		case 't':
 			slub_debug |= SLAB_TRACE;
 			break;
 		default:
@@ -918,8 +939,10 @@ static int __init setup_slub_debug(char *str)
 		}
 	}
 
+check_slabs:
 	if (*str == ',')
 		slub_debug_slabs = str + 1;
+out:
 	return 1;
 }