Re: [PATCH] io_uring: execute task_work_run() before dropping mm

[Pavel Begunkov <asml.silence@gmail.com>]

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@linux.alibaba.com>
> ---
>  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().

>  			/*
>  			 * 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