[PATCH RFC 5/8] mm, slab: convert CONFIG_SLUB_TINY checks to kmem_cache_debug()
From: Vlastimil Babka (SUSE)
Date: Wed Sep 23 2026 - 11:54:55 EST
CONFIG_SLUB_TINY shares several slowpaths with debugging as both of them
avoid percpu caching of objects. With the goal of replacing the config
with a boot-time decision, extend this sharing by hooking
slab_tiny_enabled into kmem_cache_debug() check itself.
This is made possible by the new SLAB_DEBUG_NOOP debug flag. Make
slub_debug_enabled static key always available (not just with
CONFIG_SLUB_DEBUG enabled) and have slab_tiny_enabled add
SLAB_DEBUG_NOOP to all caches in kmem_cache_flags(). Do this also in the
CONFIG_SLUB_DEBUG variant, which is currently exclusive with
CONFIG_SLUB_TINY, but that will be lifted later.
Remove the various IS_ENABLED(CONFIG_SLUB_TINY) checks that
complement kmem_cache_debug() checks as they are no longer necessary.
While removing/adjusting the CONFIG_SLUB_DEBUG #ifdefs, also convert
CONFIG_SLUB_DEBUG_ON to *_STATIC_KEY_MAYBE().
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
---
mm/slab.h | 18 ++++++------------
mm/slub.c | 38 ++++++++++++++++++--------------------
2 files changed, 24 insertions(+), 32 deletions(-)
diff --git a/mm/slab.h b/mm/slab.h
index 00d49ac0d93f..23017a5d10d1 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -520,26 +520,20 @@ struct slabinfo {
void get_slabinfo(struct kmem_cache *s, struct slabinfo *sinfo);
-#ifdef CONFIG_SLUB_DEBUG
-#ifdef CONFIG_SLUB_DEBUG_ON
-DECLARE_STATIC_KEY_TRUE(slub_debug_enabled);
-#else
-DECLARE_STATIC_KEY_FALSE(slub_debug_enabled);
-#endif
-extern void print_tracking(struct kmem_cache *s, void *object);
-long validate_slab_cache(struct kmem_cache *s);
+DECLARE_STATIC_KEY_MAYBE(CONFIG_SLUB_DEBUG_ON, slub_debug_enabled);
+
static inline bool __slub_debug_enabled(void)
{
return static_branch_unlikely(&slub_debug_enabled);
}
+
+#ifdef CONFIG_SLUB_DEBUG
+extern void print_tracking(struct kmem_cache *s, void *object);
+long validate_slab_cache(struct kmem_cache *s);
#else
static inline void print_tracking(struct kmem_cache *s, void *object)
{
}
-static inline bool __slub_debug_enabled(void)
-{
- return false;
-}
#endif
/*
diff --git a/mm/slub.c b/mm/slub.c
index 2c8477033700..c6a0422e055d 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -202,13 +202,7 @@ enum slab_flags {
#define __fastpath_inline
#endif
-#ifdef CONFIG_SLUB_DEBUG
-#ifdef CONFIG_SLUB_DEBUG_ON
-DEFINE_STATIC_KEY_TRUE(slub_debug_enabled);
-#else
-DEFINE_STATIC_KEY_FALSE(slub_debug_enabled);
-#endif
-#endif /* CONFIG_SLUB_DEBUG */
+DEFINE_STATIC_KEY_MAYBE(CONFIG_SLUB_DEBUG_ON, slub_debug_enabled);
#ifdef CONFIG_NUMA
static DEFINE_STATIC_KEY_FALSE(strict_numa);
@@ -1040,11 +1034,7 @@ static inline void *restore_red_left(struct kmem_cache *s, void *p)
/*
* Debug settings:
*/
-#if defined(CONFIG_SLUB_DEBUG_ON)
-static slab_flags_t slub_debug = DEBUG_DEFAULT_FLAGS;
-#else
-static slab_flags_t slub_debug;
-#endif
+static slab_flags_t slub_debug = IS_ENABLED(CONFIG_SLUB_DEBUG_ON) ? DEBUG_DEFAULT_FLAGS : 0;
static const char *slub_debug_string __ro_after_init;
static int disable_higher_order_debug;
@@ -2017,6 +2007,9 @@ slab_flags_t kmem_cache_flags(slab_flags_t flags, const char *name)
slab_flags_t block_flags;
slab_flags_t slub_debug_local = slub_debug;
+ if (slab_tiny_enabled)
+ flags |= SLAB_DEBUG_NOOP;
+
if (flags & SLAB_NO_USER_FLAGS)
return flags;
@@ -2087,6 +2080,9 @@ static inline void remove_full(struct kmem_cache *s, struct kmem_cache_node *n,
struct slab *slab) {}
slab_flags_t kmem_cache_flags(slab_flags_t flags, const char *name)
{
+ if (slab_tiny_enabled)
+ flags |= SLAB_DEBUG_NOOP;
+
return flags;
}
#define slub_debug 0
@@ -3942,7 +3938,7 @@ static void *get_from_partial_node(struct kmem_cache *s,
if (!pfmemalloc_match(slab, gfp_flags))
continue;
- if (IS_ENABLED(CONFIG_SLUB_TINY) || kmem_cache_debug(s)) {
+ if (kmem_cache_debug(s)) {
object = alloc_single_from_partial(s, n, slab,
ac->orig_size);
if (object)
@@ -4556,7 +4552,7 @@ static unsigned int alloc_from_new_slab(struct kmem_cache *s, struct slab *slab,
/*
* Slow path. We failed to allocate via percpu sheaves or they are not available
- * due to bootstrap or debugging enabled or SLUB_TINY.
+ * due to bootstrap or debugging enabled.
*
* We try to allocate from partial slab lists and fall back to allocating a new
* slab.
@@ -4610,7 +4606,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
stat(s, ALLOC_SLAB);
- if (IS_ENABLED(CONFIG_SLUB_TINY) || kmem_cache_debug(s)) {
+ if (kmem_cache_debug(s)) {
object = alloc_single_from_new_slab(s, slab, ac);
if (likely(object))
@@ -5764,7 +5760,7 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
unsigned long flags;
bool on_node_partial;
- if (IS_ENABLED(CONFIG_SLUB_TINY) || kmem_cache_debug(s)) {
+ if (kmem_cache_debug(s)) {
free_to_partial_list(s, slab, head, tail, cnt, addr);
return;
}
@@ -7471,7 +7467,7 @@ static bool __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags,
{
int i;
- if (IS_ENABLED(CONFIG_SLUB_TINY) || kmem_cache_debug(s)) {
+ if (kmem_cache_debug(s)) {
const struct slab_alloc_context ac = {
.caller_addr = _RET_IP_,
.orig_size = s->object_size,
@@ -7772,7 +7768,7 @@ static int init_percpu_sheaves(struct kmem_cache *s)
* cache.
*
* We keep bootstrap_sheaf for kmem_cache and kmem_cache_node,
- * caches with debug enabled, and all caches with SLUB_TINY.
+ * and caches with debugging enabled.
* For kmalloc caches it's used temporarily during the initial
* bootstrap.
*/
@@ -8588,7 +8584,7 @@ static void __init bootstrap_cache_sheaves(struct kmem_cache *s)
capacity = calculate_sheaf_capacity(s, &empty_args);
- /* capacity can be 0 due to debugging or SLUB_TINY */
+ /* capacity can be 0 due to debugging */
if (!capacity)
return;
@@ -8654,8 +8650,10 @@ void __init kmem_cache_init(void)
slab_obj_ext_has_codetag_init();
- if (slab_tiny_enabled)
+ if (slab_tiny_enabled) {
slab_max_order = 1;
+ static_branch_enable(&slub_debug_enabled);
+ }
if (slab_max_order_param <= MAX_PAGE_ORDER)
slab_max_order = slab_max_order_param;
--
2.55.0