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>

2 files changed, 23 insertions, 17 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 4344f69ae24a..8916410307f1 100644
--- a/Documentation/kernel-parameters.txt
+++ b/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]
diff --git a/Documentation/vm/slub.txt b/Documentation/vm/slub.txt
index 1523320abd87..df812b03b65d 100644
--- a/Documentation/vm/slub.txt
+++ b/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: