For certain usecases, specifically where the sample interval is always
set to a very low value such as 1ms, it can make sense to use a dynamic
branch instead of static branches due to the overhead of toggling a
static branch.
Therefore, add a new Kconfig option to remove the static branches and
instead check kfence_allocation_gate if a KFENCE allocation should be
set up.
Suggested-by: Jörn Engel <joern@xxxxxxxxxxxxxxx>
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
include/linux/kfence.h | 11 ++++++++++-
lib/Kconfig.kfence | 12 +++++++++++-
mm/kfence/core.c | 32 ++++++++++++++++++--------------
3 files changed, 39 insertions(+), 16 deletions(-)
diff --git a/include/linux/kfence.h b/include/linux/kfence.h
index 76246889ecdb..dc86b69d3903 100644
--- a/include/linux/kfence.h
+++ b/include/linux/kfence.h
@@ -4,7 +4,6 @@
#define _LINUX_KFENCE_H
#include <linux/mm.h>
-#include <linux/static_key.h>
#include <linux/types.h>
#ifdef CONFIG_KFENCE
@@ -17,7 +16,13 @@
#define KFENCE_POOL_SIZE ((CONFIG_KFENCE_NUM_OBJECTS + 1) * 2 * PAGE_SIZE)
extern char *__kfence_pool;
+#ifdef CONFIG_KFENCE_STATIC_KEYS
+#include <linux/static_key.h>
DECLARE_STATIC_KEY_FALSE(kfence_allocation_key);
+#else
+#include <linux/atomic.h>
+extern atomic_t kfence_allocation_gate;
+#endif
/**
* is_kfence_address() - check if an address belongs to KFENCE pool
@@ -104,7 +109,11 @@ void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags);
*/
static __always_inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
{
+#ifdef CONFIG_KFENCE_STATIC_KEYS
if (static_branch_unlikely(&kfence_allocation_key))
+#else
+ if (unlikely(!atomic_read(&kfence_allocation_gate)))
+#endif
return __kfence_alloc(s, size, flags);
return NULL;
}
diff --git a/lib/Kconfig.kfence b/lib/Kconfig.kfence
index d3ea24fa30fc..78f50ccb3b45 100644
--- a/lib/Kconfig.kfence
+++ b/lib/Kconfig.kfence
@@ -6,7 +6,6 @@ config HAVE_ARCH_KFENCE
menuconfig KFENCE
bool "KFENCE: low-overhead sampling-based memory safety error detector"
depends on HAVE_ARCH_KFENCE && (SLAB || SLUB)
- depends on JUMP_LABEL # To ensure performance, require jump labels
select STACKTRACE
help
KFENCE is a low-overhead sampling-based detector of heap out-of-bounds
@@ -25,6 +24,17 @@ menuconfig KFENCE
if KFENCE
+config KFENCE_STATIC_KEYS
+ bool "Use static keys to set up allocations"
+ default y
+ depends on JUMP_LABEL # To ensure performance, require jump labels
+ help
+ Use static keys (static branches) to set up KFENCE allocations. Using
+ static keys is normally recommended, because it avoids a dynamic
+ branch in the allocator's fast path. However, with very low sample
+ intervals, or on systems that do not support jump labels, a dynamic
+ branch may still be an acceptable performance trade-off.
+
config KFENCE_SAMPLE_INTERVAL
int "Default sample interval in milliseconds"
default 100
diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index f0816d5f5913..96a9a98e7453 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -88,11 +88,13 @@ struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */
+#ifdef CONFIG_KFENCE_STATIC_KEYS
/* The static key to set up a KFENCE allocation. */
DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
+#endif
/* Gates the allocation, ensuring only one succeeds in a given period. */
-static atomic_t allocation_gate = ATOMIC_INIT(1);
+atomic_t kfence_allocation_gate = ATOMIC_INIT(1);
/* Statistics counters for debugfs. */
enum kfence_counter_id {
@@ -583,29 +585,31 @@ late_initcall(kfence_debugfs_init);
static struct delayed_work kfence_timer;
static void toggle_allocation_gate(struct work_struct *work)
{
- unsigned long end_wait;
-
if (!READ_ONCE(kfence_enabled))
return;
/* Enable static key, and await allocation to happen. */
- atomic_set(&allocation_gate, 0);
+ atomic_set(&kfence_allocation_gate, 0);
+#ifdef CONFIG_KFENCE_STATIC_KEYS
static_branch_enable(&kfence_allocation_key);
/*
* Await an allocation. Timeout after 1 second, in case the kernel stops
* doing allocations, to avoid stalling this worker task for too long.
*/
- end_wait = jiffies + HZ;
- do {
- set_current_state(TASK_UNINTERRUPTIBLE);
- if (atomic_read(&allocation_gate) != 0)
- break;
- schedule_timeout(1);
- } while (time_before(jiffies, end_wait));
- __set_current_state(TASK_RUNNING);
-
+ {
+ unsigned long end_wait = jiffies + HZ;
+
+ do {
+ set_current_state(TASK_UNINTERRUPTIBLE);
+ if (atomic_read(&kfence_allocation_gate) != 0)
+ break;
+ schedule_timeout(1);
+ } while (time_before(jiffies, end_wait));
+ __set_current_state(TASK_RUNNING);
+ }
/* Disable static key and reset timer. */
static_branch_disable(&kfence_allocation_key);
+#endif
schedule_delayed_work(&kfence_timer, msecs_to_jiffies(kfence_sample_interval));
}
static DECLARE_DELAYED_WORK(kfence_timer, toggle_allocation_gate);
@@ -711,7 +715,7 @@ void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
* sense to continue writing to it and pay the associated contention
* cost, in case we have a large number of concurrent allocations.
*/
- if (atomic_read(&allocation_gate) || atomic_inc_return(&allocation_gate) > 1)
+ if (atomic_read(&kfence_allocation_gate) || atomic_inc_return(&kfence_allocation_gate) > 1)
return NULL;
if (!READ_ONCE(kfence_enabled))
--
2.30.0.284.gd98b1dd5eaa7-goog
