[PATCH] sched/fair: don't NUMA balance for kthreads

[Jens Axboe <axboe@kernel.dk>]

Stefano reported a crash with using SQPOLL with io_uring:

BUG: kernel NULL pointer dereference, address: 00000000000003b0
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 800000046c042067 P4D 800000046c042067 PUD 461fcf067 PMD 0 
Oops: 0000 [#1] SMP PTI
CPU: 2 PID: 1307 Comm: io_uring-sq Not tainted 5.7.0-rc7 #11
Hardware name: Dell Inc. PowerEdge R430/03XKDV, BIOS 1.2.6 06/08/2015
RIP: 0010:task_numa_work+0x4f/0x2c0
Code: 18 4c 8b 25 e3 f0 8e 01 49 8b 9f 00 08 00 00 4d 8b af c8 00 00 00 49 39 c7 0f 85 e8 01 00 00 48 89 6d 00 41 f6 47 24 04 75 67 <48> 8b ab b0 03 00 00 48 85 ed 75 16 8b 3d 6f 68 94 01 e8 aa fb 04
RSP: 0018:ffffaaa98415be10 EFLAGS: 00010246
RAX: ffff953ee36b8000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000001 RSI: ffff953ee36b8000 RDI: ffff953ee36b8dc8
RBP: ffff953ee36b8dc8 R08: 00000000001200db R09: ffff9542e3ad2e08
R10: ffff9542ecd20070 R11: 0000000000000000 R12: 00000000fffca35b
R13: 000000012a06a949 R14: ffff9542e3ad2c00 R15: ffff953ee36b8000
FS:  0000000000000000(0000) GS:ffff953eefc40000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000000003b0 CR3: 000000046bbd0002 CR4: 00000000001606e0
Call Trace:
 task_work_run+0x68/0xa0
 io_sq_thread+0x252/0x3d0
 ? finish_wait+0x80/0x80
 kthread+0xf9/0x130
 ? __ia32_sys_io_uring_enter+0x370/0x370
 ? kthread_park+0x90/0x90
 ret_from_fork+0x35/0x40

which is task_numa_work() oopsing on current->mm being NULL. The task
work is queued by task_tick_numa(), which checks if current->mm is NULL
at the time of the call. But this state isn't necessarily persistent,
if the kthread is using use_mm() to temporarily adopt the mm of a task.

Change the task_tick_numa() check to exclude kernel threads in general,
as it doesn't make sense to attempt ot balance for kthreads anyway.

Reported-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

This should go into 5.7 imho

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 02f323b85b6d..403556fc16d4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2908,7 +2908,7 @@ static void task_tick_numa(struct rq *rq, struct task_struct *curr)
 	/*
 	 * We don't care about NUMA placement if we don't have memory.
 	 */
-	if (!curr->mm || (curr->flags & PF_EXITING) || work->next != work)
+	if ((curr->flags & (PF_EXITING | PF_KTHREAD)) || work->next != work)
 		return;
 
 	/*

-- 
Jens Axboe