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kfence: allow use of a deferrable timer
Allow the use of a deferrable timer, which does not force CPU wake-ups when the system is idle. A consequence is that the sample interval becomes very unpredictable, to the point that it is not guaranteed that the KFENCE KUnit test still passes. Nevertheless, on power-constrained systems this may be preferable, so let's give the user the option should they accept the above trade-off. Link: https://lkml.kernel.org/r/20220308141415.3168078-1-elver@google.com Signed-off-by: Marco Elver <elver@google.com> Reviewed-by: Alexander Potapenko <glider@google.com> Cc: Dmitry Vyukov <dvyukov@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -41,6 +41,18 @@ guarded by KFENCE. The default is configurable via the Kconfig option
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``CONFIG_KFENCE_SAMPLE_INTERVAL``. Setting ``kfence.sample_interval=0``
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disables KFENCE.
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The sample interval controls a timer that sets up KFENCE allocations. By
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default, to keep the real sample interval predictable, the normal timer also
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causes CPU wake-ups when the system is completely idle. This may be undesirable
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on power-constrained systems. The boot parameter ``kfence.deferrable=1``
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instead switches to a "deferrable" timer which does not force CPU wake-ups on
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idle systems, at the risk of unpredictable sample intervals. The default is
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configurable via the Kconfig option ``CONFIG_KFENCE_DEFERRABLE``.
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.. warning::
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The KUnit test suite is very likely to fail when using a deferrable timer
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since it currently causes very unpredictable sample intervals.
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The KFENCE memory pool is of fixed size, and if the pool is exhausted, no
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further KFENCE allocations occur. With ``CONFIG_KFENCE_NUM_OBJECTS`` (default
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255), the number of available guarded objects can be controlled. Each object
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@ -45,6 +45,18 @@ config KFENCE_NUM_OBJECTS
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pages are required; with one containing the object and two adjacent
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ones used as guard pages.
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config KFENCE_DEFERRABLE
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bool "Use a deferrable timer to trigger allocations"
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help
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Use a deferrable timer to trigger allocations. This avoids forcing
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CPU wake-ups if the system is idle, at the risk of a less predictable
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sample interval.
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Warning: The KUnit test suite fails with this option enabled - due to
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the unpredictability of the sample interval!
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Say N if you are unsure.
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config KFENCE_STATIC_KEYS
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bool "Use static keys to set up allocations" if EXPERT
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depends on JUMP_LABEL
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@ -95,6 +95,10 @@ module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_inte
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static unsigned long kfence_skip_covered_thresh __read_mostly = 75;
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module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644);
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/* If true, use a deferrable timer. */
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static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE);
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module_param_named(deferrable, kfence_deferrable, bool, 0444);
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/* The pool of pages used for guard pages and objects. */
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char *__kfence_pool __read_mostly;
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EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
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@ -740,6 +744,8 @@ late_initcall(kfence_debugfs_init);
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/* === Allocation Gate Timer ================================================ */
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static struct delayed_work kfence_timer;
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#ifdef CONFIG_KFENCE_STATIC_KEYS
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/* Wait queue to wake up allocation-gate timer task. */
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static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
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@ -762,7 +768,6 @@ static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
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* avoids IPIs, at the cost of not immediately capturing allocations if the
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* instructions remain cached.
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*/
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static struct delayed_work kfence_timer;
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static void toggle_allocation_gate(struct work_struct *work)
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{
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if (!READ_ONCE(kfence_enabled))
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@ -790,7 +795,6 @@ static void toggle_allocation_gate(struct work_struct *work)
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queue_delayed_work(system_unbound_wq, &kfence_timer,
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msecs_to_jiffies(kfence_sample_interval));
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}
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static DECLARE_DELAYED_WORK(kfence_timer, toggle_allocation_gate);
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/* === Public interface ===================================================== */
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@ -809,8 +813,15 @@ static void kfence_init_enable(void)
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{
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if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
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static_branch_enable(&kfence_allocation_key);
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if (kfence_deferrable)
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INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate);
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else
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INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate);
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WRITE_ONCE(kfence_enabled, true);
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queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
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pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
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CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
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(void *)(__kfence_pool + KFENCE_POOL_SIZE));
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