[patch 212/262] kfence: always use static branches to guard kfence_alloc()

Andrew Morton
2021-11-05T20:45:49Z

From: Marco Elver <elver@google.com>
Subject: kfence: always use static branches to guard kfence_alloc()

Regardless of KFENCE mode (CONFIG_KFENCE_STATIC_KEYS: either using static
keys to gate allocations, or using a simple dynamic branch), always use a
static branch to avoid the dynamic branch in kfence_alloc() if KFENCE was
disabled at boot.

For CONFIG_KFENCE_STATIC_KEYS=n, this now avoids the dynamic branch if
KFENCE was disabled at boot.

To simplify, also unifies the location where kfence_allocation_gate is
read-checked to just be inline in kfence_alloc().

Link: https://lkml.kernel.org/r/20211019102524.2807208-1-elver@google.com
Signed-off-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

include/linux/kfence.h | 21 +++++++++++----------
mm/kfence/core.c | 16 +++++++---------
2 files changed, 18 insertions(+), 19 deletions(-)

--- a/include/linux/kfence.h~kfence-always-use-static-branches-to-guard-kfence_alloc
+++ a/include/linux/kfence.h
@@ -14,6 +14,9 @@

#ifdef CONFIG_KFENCE

+#include <linux/atomic.h>
+#include <linux/static_key.h>
+
/*
* We allocate an even number of pages, as it simplifies calculations to map
* address to metadata indices; effectively, the very first page serves as an
@@ -22,13 +25,8 @@
#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
@@ -116,13 +114,16 @@ void *__kfence_alloc(struct kmem_cache *
*/
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))
+#if defined(CONFIG_KFENCE_STATIC_KEYS) || CONFIG_KFENCE_SAMPLE_INTERVAL == 0
+ if (!static_branch_unlikely(&kfence_allocation_key))
+ return NULL;
#else
- if (unlikely(!atomic_read(&kfence_allocation_gate)))
+ if (!static_branch_likely(&kfence_allocation_key))
+ return NULL;
#endif
- return __kfence_alloc(s, size, flags);
- return NULL;
+ if (likely(atomic_read(&kfence_allocation_gate)))
+ return NULL;
+ return __kfence_alloc(s, size, flags);
}

/**
--- a/mm/kfence/core.c~kfence-always-use-static-branches-to-guard-kfence_alloc
+++ a/mm/kfence/core.c
@@ -104,10 +104,11 @@ struct kfence_metadata kfence_metadata[C
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. */
+/*
+ * The static key to set up a KFENCE allocation; or if static keys are not used
+ * to gate allocations, to avoid a load and compare if KFENCE is disabled.
+ */
DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
-#endif

/* Gates the allocation, ensuring only one succeeds in a given period. */
atomic_t kfence_allocation_gate = ATOMIC_INIT(1);
@@ -774,6 +775,8 @@ void __init kfence_init(void)
return;
}

+ if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
+ static_branch_enable(&kfence_allocation_key);
WRITE_ONCE(kfence_enabled, true);
queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
@@ -866,12 +869,7 @@ void *__kfence_alloc(struct kmem_cache *
return NULL;
}

- /*
- * allocation_gate only needs to become non-zero, so it doesn't make
- * 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(&kfence_allocation_gate) || atomic_inc_return(&kfence_allocation_gate) > 1)
+ if (atomic_inc_return(&kfence_allocation_gate) > 1)
return NULL;
#ifdef CONFIG_KFENCE_STATIC_KEYS
/*
_