Re: [PATCH for-next v3 2/9] mm/slab, slub_kunit: register kprobe to trigger _nolock APIs

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 6/15/26 13:05, Harry Yoo (Oracle) wrote:
> Since kmalloc_nolock() always fails in NMI and hardirq contexts on
> PREEMPT_RT, slub_kunit cannot properly test _nolock() APIs.
> 
> Register a kprobe pre-handler to invoke kmalloc_nolock() and
> kfree_nolock() in the middle of the slab allocator. However, do not
> register the handler on UP kernels [1].
> 
> To attach the pre-handler while s->cpu_sheaves->lock or n->list_lock
> is held, add a wrapper function for lockdep_assert_held() that calls
> a no-op function slab_attach_kprobe_locked() on debug builds. The
> function is optimized away when neither CONFIG_PROVE_LOCKING nor
> CONFIG_DEBUG_VM is selected and register_kprobe() fails.
> 
> The function calls barrier() to prevent the compiler from optimizing
> away its callsites. Otherwise, the compiler may consider the function
> does not have any side effect and remove callsites.
> 
> Link: https://lore.kernel.org/linux-mm/20260427-nolock-api-fix-v2-0-a6b83a92d9a4@kernel.org [1]
> Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>

Looks very useful!
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> ---
>  lib/tests/slub_kunit.c | 82 +++++++++++++++++++++++++++++++++++++++++++-------
>  mm/slub.c              | 36 ++++++++++++++++------
>  2 files changed, 98 insertions(+), 20 deletions(-)
> 
> diff --git a/lib/tests/slub_kunit.c b/lib/tests/slub_kunit.c
> index 11255fc8eb78..01d808cb77fa 100644
> --- a/lib/tests/slub_kunit.c
> +++ b/lib/tests/slub_kunit.c
> @@ -8,6 +8,7 @@
>  #include <linux/rcupdate.h>
>  #include <linux/delay.h>
>  #include <linux/perf_event.h>
> +#include <linux/kprobes.h>
>  #include "../mm/slab.h"
>  
>  static struct kunit_resource resource;
> @@ -292,7 +293,8 @@ static void test_krealloc_redzone_zeroing(struct kunit *test)
>  	kmem_cache_destroy(s);
>  }
>  
> -#ifdef CONFIG_PERF_EVENTS
> +#if defined(CONFIG_PERF_EVENTS) || (defined(CONFIG_KPROBES) && defined(CONFIG_SMP))
> +#define SLUB_KUNIT_TEST_KMALLOC_KFREE_NOLOCK
>  #define NR_ITERATIONS 1000
>  #define NR_OBJECTS 1000
>  static void *objects[NR_OBJECTS];
> @@ -302,10 +304,16 @@ struct test_nolock_context {
>  	int callback_count;
>  	int alloc_ok;
>  	int alloc_fail;
> +#ifdef CONFIG_PERF_EVENTS
>  	struct perf_event *event;
>  	bool is_perf_type_hw;
> +#endif
> +#ifdef CONFIG_KPROBES
> +	struct kprobe kprobe;
> +#endif
>  };
>  
> +#ifdef CONFIG_PERF_EVENTS
>  static struct perf_event_attr hw_attr = {
>  	.type = PERF_TYPE_HARDWARE,
>  	.config = PERF_COUNT_HW_CPU_CYCLES,
> @@ -326,13 +334,10 @@ static struct perf_event_attr sw_attr = {
>  	.sample_freq = 100000,
>  };
>  
> -static void overflow_handler_test_nolock(struct perf_event *event,
> -					 struct perf_sample_data *data,
> -					 struct pt_regs *regs)
> +static void test_nolock(struct test_nolock_context *ctx)
>  {
>  	void *objp;
>  	gfp_t gfp;
> -	struct test_nolock_context *ctx = event->overflow_handler_context;
>  
>  	/* __GFP_ACCOUNT to test kmalloc_nolock() in alloc_slab_obj_exts() */
>  	gfp = (ctx->callback_count % 2) ? 0 : __GFP_ACCOUNT;
> @@ -347,6 +352,15 @@ static void overflow_handler_test_nolock(struct perf_event *event,
>  	ctx->callback_count++;
>  }
>  
> +static void overflow_handler_test_nolock(struct perf_event *event,
> +					 struct perf_sample_data *data,
> +					 struct pt_regs *regs)
> +{
> +	struct test_nolock_context *ctx = event->overflow_handler_context;
> +
> +	test_nolock(ctx);
> +}
> +
>  static bool enable_perf_events(struct test_nolock_context *ctx)
>  {
>  	struct perf_event *event;
> @@ -382,17 +396,60 @@ static void disable_perf_events(struct test_nolock_context *ctx)
>  	perf_event_disable(ctx->event);
>  	perf_event_release_kernel(ctx->event);
>  }
> +#else
> +static bool enable_perf_events(struct test_nolock_context *ctx) { return false; }
> +static void disable_perf_events(struct test_nolock_context *ctx) { }
> +#endif
> +
> +#if defined(CONFIG_KPROBES) && defined(CONFIG_SMP)
> +static int slab_kprobe_pre_handler(struct kprobe *p, struct pt_regs *regs)
> +{
> +	struct test_nolock_context *ctx;
> +
> +	ctx = container_of(p, struct test_nolock_context, kprobe);
> +	test_nolock(ctx);
> +	return 0;
> +}
> +
> +static bool register_slab_kprobes(struct test_nolock_context *ctx)
> +{
> +	ctx->kprobe.symbol_name = "slab_attach_kprobe_locked";
> +	ctx->kprobe.pre_handler = slab_kprobe_pre_handler;
> +
> +	if (register_kprobe(&ctx->kprobe))
> +		return false;
> +	return true;
> +}
> +
> +static void unregister_slab_kprobes(struct test_nolock_context *ctx)
> +{
> +	kunit_info(ctx->test, "kprobes: callback_count: %d, alloc_ok: %d, alloc_fail: %d\n",
> +		   ctx->callback_count, ctx->alloc_ok, ctx->alloc_fail);
> +	unregister_kprobe(&ctx->kprobe);
> +}
> +#else
> +static bool register_slab_kprobes(struct test_nolock_context *ctx) { return false; }
> +static void unregister_slab_kprobes(struct test_nolock_context *ctx) { }
> +#endif
>  
>  static void test_kmalloc_kfree_nolock(struct kunit *test)
>  {
>  	int i, j;
> -	struct test_nolock_context ctx = { .test = test };
> +	struct test_nolock_context perf_ctx = { .test = test };
> +	struct test_nolock_context kprobe_ctx = { .test = test };
>  	bool alloc_fail = false;
>  	bool perf_events_enabled;
> +	bool slab_kprobes_enabled;
>  
> -	perf_events_enabled = enable_perf_events(&ctx);
> -	if (!perf_events_enabled)
> -		kunit_skip(test, "Failed to create perf event");
> +	perf_events_enabled = enable_perf_events(&perf_ctx);
> +	slab_kprobes_enabled = register_slab_kprobes(&kprobe_ctx);
> +
> +	if (!perf_events_enabled && !slab_kprobes_enabled)
> +		kunit_skip(test, "Failed to enable perf event and kprobe, skipping");
> +	else if (!perf_events_enabled)
> +		kunit_info(test, "Failed to create perf event");
> +	if (!slab_kprobes_enabled)
> +		kunit_info(test, "Failed to register kprobe pre-handler");
>  
>  	for (i = 0; i < NR_ITERATIONS; i++) {
>  		for (j = 0; j < NR_OBJECTS; j++) {
> @@ -412,7 +469,10 @@ static void test_kmalloc_kfree_nolock(struct kunit *test)
>  	}
>  
>  cleanup:
> -	disable_perf_events(&ctx);
> +	if (perf_events_enabled)
> +		disable_perf_events(&perf_ctx);
> +	if (slab_kprobes_enabled)
> +		unregister_slab_kprobes(&kprobe_ctx);
>  
>  	if (alloc_fail)
>  		kunit_skip(test, "Allocation failed");
> @@ -444,7 +504,7 @@ static struct kunit_case test_cases[] = {
>  	KUNIT_CASE(test_kfree_rcu_wq_destroy),
>  	KUNIT_CASE(test_leak_destroy),
>  	KUNIT_CASE(test_krealloc_redzone_zeroing),
> -#ifdef CONFIG_PERF_EVENTS
> +#ifdef SLUB_KUNIT_TEST_KMALLOC_KFREE_NOLOCK
>  	KUNIT_CASE_SLOW(test_kmalloc_kfree_nolock),
>  #endif
>  	{}
> diff --git a/mm/slub.c b/mm/slub.c
> index 813fb863254d..87ca154ccd80 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -908,6 +908,24 @@ static inline unsigned int obj_exts_offset_in_object(struct kmem_cache *s)
>  }
>  #endif
>  
> +/*
> + * A no-op function used to attach kprobe handlers in slub_kunit tests.
> + * The barrier is needed to prevent the compiler from optimizing out callsites.
> + */
> +#if defined(CONFIG_DEBUG_VM) || defined(CONFIG_PROVE_LOCKING)
> +static noinline void slab_attach_kprobe_locked(void)
> +{
> +	barrier();
> +}
> +#else
> +static inline void slab_attach_kprobe_locked(void) { }
> +#endif
> +
> +#define slab_lockdep_assert_held(lock) do {	\
> +	lockdep_assert_held(lock);		\
> +	slab_attach_kprobe_locked();	\
> +} while (0)
> +
>  #ifdef CONFIG_SLUB_DEBUG
>  
>  /*
> @@ -1665,7 +1683,7 @@ static void add_full(struct kmem_cache *s,
>  	if (!(s->flags & SLAB_STORE_USER))
>  		return;
>  
> -	lockdep_assert_held(&n->list_lock);
> +	slab_lockdep_assert_held(&n->list_lock);
>  	list_add(&slab->slab_list, &n->full);
>  }
>  
> @@ -1674,7 +1692,7 @@ static void remove_full(struct kmem_cache *s, struct kmem_cache_node *n, struct
>  	if (!(s->flags & SLAB_STORE_USER))
>  		return;
>  
> -	lockdep_assert_held(&n->list_lock);
> +	slab_lockdep_assert_held(&n->list_lock);
>  	list_del(&slab->slab_list);
>  }
>  
> @@ -2866,7 +2884,7 @@ static unsigned int __sheaf_flush_main_batch(struct kmem_cache *s)
>  	void *objects[PCS_BATCH_MAX];
>  	struct slab_sheaf *sheaf;
>  
> -	lockdep_assert_held(this_cpu_ptr(&s->cpu_sheaves->lock));
> +	slab_lockdep_assert_held(this_cpu_ptr(&s->cpu_sheaves->lock));
>  
>  	pcs = this_cpu_ptr(s->cpu_sheaves);
>  	sheaf = pcs->main;
> @@ -3545,7 +3563,7 @@ __add_partial(struct kmem_cache_node *n, struct slab *slab, enum add_mode mode)
>  static inline void add_partial(struct kmem_cache_node *n,
>  				struct slab *slab, enum add_mode mode)
>  {
> -	lockdep_assert_held(&n->list_lock);
> +	slab_lockdep_assert_held(&n->list_lock);
>  	__add_partial(n, slab, mode);
>  }
>  
> @@ -3559,7 +3577,7 @@ static inline void clear_node_partial_state(struct kmem_cache_node *n,
>  static inline void remove_partial(struct kmem_cache_node *n,
>  					struct slab *slab)
>  {
> -	lockdep_assert_held(&n->list_lock);
> +	slab_lockdep_assert_held(&n->list_lock);
>  	list_del(&slab->slab_list);
>  	clear_node_partial_state(n, slab);
>  }
> @@ -3575,7 +3593,7 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
>  {
>  	void *object;
>  
> -	lockdep_assert_held(&n->list_lock);
> +	slab_lockdep_assert_held(&n->list_lock);
>  
>  #ifdef CONFIG_SLUB_DEBUG
>  	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
> @@ -4646,7 +4664,7 @@ __pcs_replace_empty_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs,
>  	struct node_barn *barn;
>  	bool allow_spin;
>  
> -	lockdep_assert_held(this_cpu_ptr(&s->cpu_sheaves->lock));
> +	slab_lockdep_assert_held(this_cpu_ptr(&s->cpu_sheaves->lock));
>  
>  	/* Bootstrap or debug cache, back off */
>  	if (unlikely(!cache_has_sheaves(s))) {
> @@ -5786,7 +5804,7 @@ static void __pcs_install_empty_sheaf(struct kmem_cache *s,
>  		struct slub_percpu_sheaves *pcs, struct slab_sheaf *empty,
>  		struct node_barn *barn)
>  {
> -	lockdep_assert_held(this_cpu_ptr(&s->cpu_sheaves->lock));
> +	slab_lockdep_assert_held(this_cpu_ptr(&s->cpu_sheaves->lock));
>  
>  	/* This is what we expect to find if nobody interrupted us. */
>  	if (likely(!pcs->spare)) {
> @@ -5837,7 +5855,7 @@ __pcs_replace_full_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs,
>  	bool put_fail;
>  
>  restart:
> -	lockdep_assert_held(this_cpu_ptr(&s->cpu_sheaves->lock));
> +	slab_lockdep_assert_held(this_cpu_ptr(&s->cpu_sheaves->lock));
>  
>  	/* Bootstrap or debug cache, back off */
>  	if (unlikely(!cache_has_sheaves(s))) {
>