Commit fa5ec8a1 authored by David Rientjes's avatar David Rientjes Committed by Pekka Enberg
Browse files

slub: add option to disable higher order debugging slabs



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.

Reported-by: default avatarLarry Finger <Larry.Finger@lwfinger.net>
Tested-by: default avatarLarry Finger <Larry.Finger@lwfinger.net>
Cc: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: default avatarDavid Rientjes <rientjes@google.com>
Signed-off-by: default avatarPekka Enberg <penberg@cs.helsinki.fi>
parent c2cc49a2
......@@ -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:
......
......@@ -141,6 +141,13 @@
#define DEBUG_DEFAULT_FLAGS (SLAB_DEBUG_FREE | SLAB_RED_ZONE | \
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
*/
......@@ -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;
......
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