Commit 73ab05aa authored by Waiman Long's avatar Waiman Long Committed by Peter Zijlstra
Browse files

sched/core: Disable page allocation in task_tick_mm_cid() 



With KASAN and PREEMPT_RT enabled, calling task_work_add() in
task_tick_mm_cid() may cause the following splat.

[   63.696416] BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
[   63.696416] in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 610, name: modprobe
[   63.696416] preempt_count: 10001, expected: 0
[   63.696416] RCU nest depth: 1, expected: 1

This problem is caused by the following call trace.

  sched_tick() [ acquire rq->__lock ]
   -> task_tick_mm_cid()
    -> task_work_add()
     -> __kasan_record_aux_stack()
      -> kasan_save_stack()
       -> stack_depot_save_flags()
        -> alloc_pages_mpol_noprof()
         -> __alloc_pages_noprof()
	  -> get_page_from_freelist()
	   -> rmqueue()
	    -> rmqueue_pcplist()
	     -> __rmqueue_pcplist()
	      -> rmqueue_bulk()
	       -> rt_spin_lock()

The rq lock is a raw_spinlock_t. We can't sleep while holding
it. IOW, we can't call alloc_pages() in stack_depot_save_flags().

The task_tick_mm_cid() function with its task_work_add() call was
introduced by commit 223baf9d ("sched: Fix performance regression
introduced by mm_cid") in v6.4 kernel.

Fortunately, there is a kasan_record_aux_stack_noalloc() variant that
calls stack_depot_save_flags() while not allowing it to allocate
new pages.  To allow task_tick_mm_cid() to use task_work without
page allocation, a new TWAF_NO_ALLOC flag is added to enable calling
kasan_record_aux_stack_noalloc() instead of kasan_record_aux_stack()
if set. The task_tick_mm_cid() function is modified to add this new flag.

The possible downside is the missing stack trace in a KASAN report due
to new page allocation required when task_work_add_noallloc() is called
which should be rare.

Fixes: 223baf9d ("sched: Fix performance regression introduced by mm_cid")
Signed-off-by: default avatarWaiman Long <longman@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20241010014432.194742-1-longman@redhat.com
parent d16b7eb6

include/linux/task_work.h

+4 −1
Original line number Diff line number Diff line
@@ -14,11 +14,14 @@ init_task_work(struct callback_head *twork, task_work_func_t func) 
}



enum task_work_notify_mode {

	TWA_NONE,

	TWA_NONE = 0,

	TWA_RESUME,

	TWA_SIGNAL,

	TWA_SIGNAL_NO_IPI,

	TWA_NMI_CURRENT,



	TWA_FLAGS = 0xff00,

	TWAF_NO_ALLOC = 0x0100,

};



static inline bool task_work_pending(struct task_struct *task)

kernel/sched/core.c

+3 −1
Original line number Diff line number Diff line
@@ -10458,7 +10458,9 @@ void task_tick_mm_cid(struct rq *rq, struct task_struct *curr) 
		return;

	if (time_before(now, READ_ONCE(curr->mm->mm_cid_next_scan)))

		return;

	task_work_add(curr, work, TWA_RESUME);



	/* No page allocation under rq lock */

	task_work_add(curr, work, TWA_RESUME | TWAF_NO_ALLOC);

}



void sched_mm_cid_exit_signals(struct task_struct *t)

kernel/task_work.c

+13 −2
Original line number Diff line number Diff line
@@ -55,14 +55,25 @@ int task_work_add(struct task_struct *task, struct callback_head *work, 
		  enum task_work_notify_mode notify)

{

	struct callback_head *head;

	int flags = notify & TWA_FLAGS;



	notify &= ~TWA_FLAGS;

	if (notify == TWA_NMI_CURRENT) {

		if (WARN_ON_ONCE(task != current))

			return -EINVAL;

		if (!IS_ENABLED(CONFIG_IRQ_WORK))

			return -EINVAL;

	} else {

		/* record the work call stack in order to print it in KASAN reports */

		/*

		 * Record the work call stack in order to print it in KASAN

		 * reports.

		 *

		 * Note that stack allocation can fail if TWAF_NO_ALLOC flag

		 * is set and new page is needed to expand the stack buffer.

		 */

		if (flags & TWAF_NO_ALLOC)

			kasan_record_aux_stack_noalloc(work);

		else

			kasan_record_aux_stack(work);

	}