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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>
This commit is contained in:
Marco Elver 2022-03-22 14:48:25 -07:00 committed by Linus Torvalds
parent 3cb1c9620e
commit 737b6a10ac
3 changed files with 37 additions and 2 deletions
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12
Documentation/dev-tools/kfence.rst
View File

@ -41,6 +41,18 @@ guarded by KFENCE. The default is configurable via the Kconfig option
``CONFIG_KFENCE_SAMPLE_INTERVAL``. Setting ``kfence.sample_interval=0``
disables KFENCE.
The sample interval controls a timer that sets up KFENCE allocations. By
default, to keep the real sample interval predictable, the normal timer also
causes CPU wake-ups when the system is completely idle. This may be undesirable
on power-constrained systems. The boot parameter ``kfence.deferrable=1``
instead switches to a "deferrable" timer which does not force CPU wake-ups on
idle systems, at the risk of unpredictable sample intervals. The default is
configurable via the Kconfig option ``CONFIG_KFENCE_DEFERRABLE``.
.. warning::
The KUnit test suite is very likely to fail when using a deferrable timer
since it currently causes very unpredictable sample intervals.
The KFENCE memory pool is of fixed size, and if the pool is exhausted, no
further KFENCE allocations occur. With ``CONFIG_KFENCE_NUM_OBJECTS`` (default
255), the number of available guarded objects can be controlled. Each object

12
lib/Kconfig.kfence
View File

@ -45,6 +45,18 @@ config KFENCE_NUM_OBJECTS
pages are required; with one containing the object and two adjacent
ones used as guard pages.
config KFENCE_DEFERRABLE
bool "Use a deferrable timer to trigger allocations"
help
Use a deferrable timer to trigger allocations. This avoids forcing
CPU wake-ups if the system is idle, at the risk of a less predictable
sample interval.
Warning: The KUnit test suite fails with this option enabled - due to
the unpredictability of the sample interval!
Say N if you are unsure.
config KFENCE_STATIC_KEYS
bool "Use static keys to set up allocations" if EXPERT
depends on JUMP_LABEL

15
mm/kfence/core.c
View File

@ -95,6 +95,10 @@ module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_inte
static unsigned long kfence_skip_covered_thresh __read_mostly = 75;
module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644);
/* If true, use a deferrable timer. */
static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE);
module_param_named(deferrable, kfence_deferrable, bool, 0444);
/* The pool of pages used for guard pages and objects. */
char *__kfence_pool __read_mostly;
EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
@ -740,6 +744,8 @@ late_initcall(kfence_debugfs_init);
/* === Allocation Gate Timer ================================================ */
static struct delayed_work kfence_timer;
#ifdef CONFIG_KFENCE_STATIC_KEYS
/* Wait queue to wake up allocation-gate timer task. */
static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
@ -762,7 +768,6 @@ static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
* avoids IPIs, at the cost of not immediately capturing allocations if the
* instructions remain cached.
*/
static struct delayed_work kfence_timer;
static void toggle_allocation_gate(struct work_struct *work)
{
if (!READ_ONCE(kfence_enabled))
@ -790,7 +795,6 @@ static void toggle_allocation_gate(struct work_struct *work)
queue_delayed_work(system_unbound_wq, &kfence_timer,
msecs_to_jiffies(kfence_sample_interval));
}
static DECLARE_DELAYED_WORK(kfence_timer, toggle_allocation_gate);
/* === Public interface ===================================================== */
@ -809,8 +813,15 @@ static void kfence_init_enable(void)
{
if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
static_branch_enable(&kfence_allocation_key);
if (kfence_deferrable)
INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate);
else
INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate);
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,
CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
(void *)(__kfence_pool + KFENCE_POOL_SIZE));