On 06/06/2020 18:55, Pavel Begunkov wrote:
> On 06/06/2020 18:12, Xiaoguang Wang wrote:
>> While testing io_uring in our internal kernel, note it's not upstream
>> kernel, we see below panic:
>> [ 872.498723] x29: ffff00002d553cf0 x28: 0000000000000000
>> [ 872.508973] x27: ffff807ef691a0e0 x26: 0000000000000000
>> [ 872.519116] x25: 0000000000000000 x24: ffff0000090a7980
>> [ 872.529184] x23: ffff000009272060 x22: 0000000100022b11
>> [ 872.539144] x21: 0000000046aa5668 x20: ffff80bee8562b18
>> [ 872.549000] x19: ffff80bee8562080 x18: 0000000000000000
>> [ 872.558876] x17: 0000000000000000 x16: 0000000000000000
>> [ 872.568976] x15: 0000000000000000 x14: 0000000000000000
>> [ 872.578762] x13: 0000000000000000 x12: 0000000000000000
>> [ 872.588474] x11: 0000000000000000 x10: 0000000000000c40
>> [ 872.598324] x9 : ffff000008100c00 x8 : 000000007ffff000
>> [ 872.608014] x7 : ffff80bee8562080 x6 : ffff80beea862d30
>> [ 872.617709] x5 : 0000000000000000 x4 : ffff80beea862d48
>> [ 872.627399] x3 : ffff80bee8562b18 x2 : 0000000000000000
>> [ 872.637044] x1 : ffff0000090a7000 x0 : 0000000000208040
>> [ 872.646575] Call trace:
>> [ 872.653139] task_numa_work+0x4c/0x310
>> [ 872.660916] task_work_run+0xb0/0xe0
>> [ 872.668400] io_sq_thread+0x164/0x388
>> [ 872.675829] kthread+0x108/0x138
>>
>> The reason is that once io_sq_thread has a valid mm, schedule subsystem
>> may call task_tick_numa() adding a task_numa_work() callback, which will
>> visit mm, then above panic will happen.
>>
>> To fix this bug, only call task_work_run() before dropping mm.
>
> So, the problem is that poll/async paths re-issue requests with
> __io_queue_sqe(), which doesn't care about current->mm, and which
> can be NULL for io_sq_thread(). Right?
>
>>
>> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@xxxxxxxxxxxxxxxxx>
>> ---
>> fs/io_uring.c | 15 ++++++++-------
>> 1 file changed, 8 insertions(+), 7 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 6391a00ff8b7..32381984b2a6 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -6134,6 +6134,13 @@ static int io_sq_thread(void *data)
>> * to enter the kernel to reap and flush events.
>> */
>> if (!to_submit || ret == -EBUSY) {
>> + /*
>> + * Current task context may already have valid mm, that
>> + * means some works that visit mm may have been queued,
>> + * so we must execute the works before dropping mm.
>> + */
>> + if (current->task_works)
>> + task_work_run();
>
> Even though you're not dropping mm, the thread might not have it in the first
> place. see how it's done in io_init_req(). How about setting mm either lazily
> in io_poll_task_func()/io_async_task_func(), or before task_work_run() in
> io_sq_thread().
Thinking about use_mm(), it's more about setting up env before execution rather
than request intialisation. Another way would be to move use_mm() from io_init_req()
into __io_queue_sqe(), more clearly separating responsibilities.
BTW, it may need adding extra io_sq_thread_drop_mm() either way
>
>> /*
>> * Drop cur_mm before scheduling, we can't hold it for
>> * long periods (or over schedule()). Do this before
>> @@ -6152,8 +6159,6 @@ static int io_sq_thread(void *data)
>> if (!list_empty(&ctx->poll_list) ||
>> (!time_after(jiffies, timeout) && ret != -EBUSY &&
>> !percpu_ref_is_dying(&ctx->refs))) {
>> - if (current->task_works)
>> - task_work_run();
>> cond_resched();
>> continue;
>> }
>> @@ -6185,11 +6190,7 @@ static int io_sq_thread(void *data)
>> finish_wait(&ctx->sqo_wait, &wait);
>> break;
>> }
>> - if (current->task_works) {
>> - task_work_run();
>> - finish_wait(&ctx->sqo_wait, &wait);
>> - continue;
>> - }
>> +
>> if (signal_pending(current))
>> flush_signals(current);
>> schedule();
>>
>
--
Pavel Begunkov
