Re: [PATCH v2 1/2] io_uring: Add new functions to handle user fault scenarios

[Pavel Begunkov <asml.silence@xxxxxxxxx>]

On 4/23/25 16:55, Jens Axboe wrote:
> Something like this, perhaps - it'll ensure that io-wq workers get a
> chance to flush out pending work, which should prevent the looping. I've
> attached a basic test case. It'll issue a write that will fault, and
> then try and cancel that as a way to trigger the TIF_NOTIFY_SIGNAL based
> looping.
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index d80f94346199..e18926dbf20a 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -32,6 +32,7 @@
>   #include <linux/swapops.h>
>   #include <linux/miscdevice.h>
>   #include <linux/uio.h>
> +#include <linux/io_uring.h>
>   
>   static int sysctl_unprivileged_userfaultfd __read_mostly;
>   
> @@ -376,6 +377,8 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>   	 */
>   	if (current->flags & (PF_EXITING|PF_DUMPCORE))
>   		goto out;
> +	else if (current->flags & PF_IO_WORKER)
> +		io_worker_fault();
>   
>   	assert_fault_locked(vmf);
>   
> diff --git a/include/linux/io_uring.h b/include/linux/io_uring.h
> index 85fe4e6b275c..d93dd7402a28 100644
> --- a/include/linux/io_uring.h
> +++ b/include/linux/io_uring.h
> @@ -28,6 +28,7 @@ static inline void io_uring_free(struct task_struct *tsk)
>   	if (tsk->io_uring)
>   		__io_uring_free(tsk);
>   }
> +void io_worker_fault(void);
>   #else
>   static inline void io_uring_task_cancel(void)
>   {
> @@ -46,6 +47,9 @@ static inline bool io_is_uring_fops(struct file *file)
>   {
>   	return false;
>   }
> +static inline void io_worker_fault(void)
> +{
> +}
>   #endif
>   
>   #endif
> diff --git a/io_uring/io-wq.c b/io_uring/io-wq.c
> index d52069b1177b..f74bea028ec7 100644
> --- a/io_uring/io-wq.c
> +++ b/io_uring/io-wq.c
> @@ -1438,3 +1438,13 @@ static __init int io_wq_init(void)
>   	return 0;
>   }
>   subsys_initcall(io_wq_init);
> +
> +void io_worker_fault(void)
> +{
> +	if (test_thread_flag(TIF_NOTIFY_SIGNAL))
> +		clear_notify_signal();
> +	if (test_thread_flag(TIF_NOTIFY_RESUME))
> +		resume_user_mode_work(NULL);
> +	if (task_work_pending(current))
> +		task_work_run();

Looking at the stacktrace, that sounds dangerous

iou-wrk-44588  [kernel.kallsyms]  [k] io_wq_worker
iou-wrk-44588  [kernel.kallsyms]  [k] io_worker_handle_work
iou-wrk-44588  [kernel.kallsyms]  [k] io_wq_submit_work
iou-wrk-44588  [kernel.kallsyms]  [k] io_issue_sqe
iou-wrk-44588  [kernel.kallsyms]  [k] io_write
iou-wrk-44588  [kernel.kallsyms]  [k] blkdev_write_iter
iou-wrk-44588  [kernel.kallsyms]  [k] iomap_file_buffered_write
iou-wrk-44588  [kernel.kallsyms]  [k] iomap_write_iter
iou-wrk-44588  [kernel.kallsyms]  [k] fault_in_iov_iter_readable
iou-wrk-44588  [kernel.kallsyms]  [k] fault_in_readable
iou-wrk-44588  [kernel.kallsyms]  [k] asm_exc_page_fault
iou-wrk-44588  [kernel.kallsyms]  [k] exc_page_fault
iou-wrk-44588  [kernel.kallsyms]  [k] do_user_addr_fault
iou-wrk-44588  [kernel.kallsyms]  [k] handle_mm_fault
iou-wrk-44588  [kernel.kallsyms]  [k] hugetlb_fault
iou-wrk-44588  [kernel.kallsyms]  [k] hugetlb_no_page
iou-wrk-44588  [kernel.kallsyms]  [k] hugetlb_handle_userfault
iou-wrk-44588  [kernel.kallsyms]  [k] handle_userfault

It might be holding a good bunch of locks, and then it's trapped
in a page fault handler. Do normal / non-PF_IO_WORKER tasks run
task_work from handle_userfault?

--
Pavel Begunkov