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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

Documentation/vm/slub.txt

+10 −0
Original line number Diff line number Diff line
@@ -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:

mm/slub.c

+38 −3
Original line number Diff line number Diff line
@@ -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;