[patch v2] slub: add option to disable higher order debugging slabs

David Rientjes
2009-07-07T07:14:14Z

When debugging is enabled, slub requires that additional metadata be
stored in slabs for certain options: SLAB_RED_ZONE, SLAB_POISON, and
SLAB_STORE_USER.

Consequently, it may require that the minimum possible slab order needed
to allocate a single object be greater when using these options. The
most notable example is for objects that are PAGE_SIZE bytes in size.

Higher minimum slab orders may cause page allocation failures when oom or
under heavy fragmentation.

This patch adds a new slub_debug option, which disables debugging by
default for caches that would have resulted in higher minimum orders:

slub_debug=O

When this option is used on systems with 4K pages, kmalloc-4096, for
example, will not have debugging enabled by default even if
CONFIG_SLUB_DEBUG_ON is defined because it would have resulted in a
order-1 minimum slab order.

Cc: Christoph Lameter <cl-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Signed-off-by: David Rientjes <rientjes-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
---

V1 -> V2: Removed spurious `goto out'.

Documentation/vm/slub.txt | 10 ++++++++++
mm/slub.c | 41 ++++++++++++++++++++++++++++++++++++++---
2 files changed, 48 insertions(+), 3 deletions(-)

diff --git a/Documentation/vm/slub.txt b/Documentation/vm/slub.txt
--- 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)
+ O Switch debugging off for caches that would have
+ caused higher minimum slab orders
- Switch all debugging off (useful if the kernel is
configured with CONFIG_SLUB_DEBUG_ON)

@@ -59,6 +61,14 @@ to the dentry cache with

slub_debug=F,dentry

+Debugging options may require the minimum possible slab order to increase as
+a result of storing the metadata (for example, caches with PAGE_SIZE object
+sizes). This has a higher liklihood of resulting in slab allocation errors
+in low memory situations or if there's high fragmentation of memory. To
+switch off debugging for such caches by default, use
+
+ slub_debug=O
+
In case you forgot to enable debugging on the kernel command line: It is
possible to enable debugging manually when the kernel is up. Look at the
contents of:
diff --git a/mm/slub.c b/mm/slub.c
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -142,6 +142,13 @@
SLAB_POISON | SLAB_STORE_USER)

/*
+ * Debugging flags that require metadata to be stored in the slab, up to
+ * DEBUG_SIZE in size.
+ */
+#define DEBUG_SIZE_FLAGS (SLAB_RED_ZONE | SLAB_POISON | SLAB_STORE_USER)
+#define DEBUG_SIZE (3 * sizeof(void *) + 2 * sizeof(struct track))
+
+/*
* Set of flags that will prevent slab merging
*/
#define SLUB_NEVER_MERGE (SLAB_RED_ZONE | SLAB_POISON | SLAB_STORE_USER | \
@@ -326,6 +333,7 @@ static int slub_debug;
#endif

static char *slub_debug_slabs;
+static int disable_higher_order_debug;

/*
* Object debugging
@@ -977,6 +985,15 @@ static int __init setup_slub_debug(char *str)
*/
goto check_slabs;

+ if (tolower(*str) == 'o') {
+ /*
+ * Avoid enabling debugging on caches if its minimum order
+ * would increase as a result.
+ */
+ disable_higher_order_debug = 1;
+ goto out;
+ }
+
slub_debug = 0;
if (*str == '-')
/*
@@ -1023,13 +1040,27 @@ static unsigned long kmem_cache_flags(unsigned long objsize,
unsigned long flags, const char *name,
void (*ctor)(void *))
{
+ int debug_flags = slub_debug;
+
/*
* Enable debugging if selected on the kernel commandline.
*/
- if (slub_debug && (!slub_debug_slabs ||
- strncmp(slub_debug_slabs, name, strlen(slub_debug_slabs)) == 0))
- flags |= slub_debug;
+ if (debug_flags) {
+ if (slub_debug_slabs &&
+ strncmp(slub_debug_slabs, name, strlen(slub_debug_slabs)))
+ goto out;
+
+ /*
+ * Disable debugging that increases slab size if the minimum
+ * slab order would have increased as a result.
+ */
+ if (disable_higher_order_debug &&
+ get_order(objsize + DEBUG_SIZE) > get_order(objsize))
+ debug_flags &= ~DEBUG_SIZE_FLAGS;

+ flags |= debug_flags;
+ }
+out:
return flags;
}
#else
@@ -1561,6 +1592,10 @@ slab_out_of_memory(struct kmem_cache *s, gfp_t gfpflags, int nid)
"default order: %d, min order: %d\n", s->name, s->objsize,
s->size, oo_order(s->oo), oo_order(s->min));

+ if (oo_order(s->min) > get_order(s->objsize))
+ printk(KERN_WARNING " %s debugging increased min order, use "
+ "slub_debug=O to disable.\n", s->name);
+
for_each_online_node(node) {
struct kmem_cache_node *n = get_node(s, node);
unsigned long nr_slabs;