Message-ID: <CANHzP_vi1SaC+jP_UZqsjFA=Gu=Q3ST0XR_ECm=4O-5G8Jmqqg@mail.gmail.com>
Date: Thu, 24 Apr 2025 23:12:49 +0800
From: 姜智伟 <qq282012236@...il.com>
To: Jens Axboe <axboe@...nel.dk>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz, 
	akpm@...ux-foundation.org, peterx@...hat.com, asml.silence@...il.com, 
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, io-uring@...r.kernel.org
Subject: Re: [PATCH v2 1/2] io_uring: Add new functions to handle user fault scenarios

Jens Axboe <axboe@...nel.dk> 于2025年4月24日周四 22:53写道：
>
> On 4/24/25 8:45 AM, ??? wrote:
> > Jens Axboe <axboe@...nel.dk> ?2025?4?24??? 22:13???
> >>
> >> On 4/24/25 8:08 AM, ??? wrote:
> >>> Jens Axboe <axboe@...nel.dk> ?2025?4?24??? 06:58???
> >>>>
> >>>> On 4/23/25 9:55 AM, 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.
> >>>>
> >>>> Something that may actually work - use TASK_UNINTERRUPTIBLE IFF
> >>>> signal_pending() is true AND the fault has already been tried once
> >>>> before. If that's the case, rather than just call schedule() with
> >>>> TASK_INTERRUPTIBLE, use TASK_UNINTERRUPTIBLE and schedule_timeout() with
> >>>> a suitable timeout length that prevents the annoying parts busy looping.
> >>>> I used HZ / 10.
> >>>>
> >>>> I don't see how to fix userfaultfd for this case, either using io_uring
> >>>> or normal write(2). Normal syscalls can pass back -ERESTARTSYS and get
> >>>> it retried, but there's no way to do that from inside fault handling. So
> >>>> I think we just have to be nicer about it.
> >>>>
> >>>> Andrew, as the userfaultfd maintainer, what do you think?
> >>>>
> >>>> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> >>>> index d80f94346199..1016268c7b51 100644
> >>>> --- a/fs/userfaultfd.c
> >>>> +++ b/fs/userfaultfd.c
> >>>> @@ -334,15 +334,29 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
> >>>>         return ret;
> >>>>  }
> >>>>
> >>>> -static inline unsigned int userfaultfd_get_blocking_state(unsigned int flags)
> >>>> +struct userfault_wait {
> >>>> +       unsigned int task_state;
> >>>> +       bool timeout;
> >>>> +};
> >>>> +
> >>>> +static struct userfault_wait userfaultfd_get_blocking_state(unsigned int flags)
> >>>>  {
> >>>> +       /*
> >>>> +        * If the fault has already been tried AND there's a signal pending
> >>>> +        * for this task, use TASK_UNINTERRUPTIBLE with a small timeout.
> >>>> +        * This prevents busy looping where schedule() otherwise does nothing
> >>>> +        * for TASK_INTERRUPTIBLE when the task has a signal pending.
> >>>> +        */
> >>>> +       if ((flags & FAULT_FLAG_TRIED) && signal_pending(current))
> >>>> +               return (struct userfault_wait) { TASK_UNINTERRUPTIBLE, true };
> >>>> +
> >>>>         if (flags & FAULT_FLAG_INTERRUPTIBLE)
> >>>> -               return TASK_INTERRUPTIBLE;
> >>>> +               return (struct userfault_wait) { TASK_INTERRUPTIBLE, false };
> >>>>
> >>>>         if (flags & FAULT_FLAG_KILLABLE)
> >>>> -               return TASK_KILLABLE;
> >>>> +               return (struct userfault_wait) { TASK_KILLABLE, false };
> >>>>
> >>>> -       return TASK_UNINTERRUPTIBLE;
> >>>> +       return (struct userfault_wait) { TASK_UNINTERRUPTIBLE, false };
> >>>>  }
> >>>>
> >>>>  /*
> >>>> @@ -368,7 +382,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> >>>>         struct userfaultfd_wait_queue uwq;
> >>>>         vm_fault_t ret = VM_FAULT_SIGBUS;
> >>>>         bool must_wait;
> >>>> -       unsigned int blocking_state;
> >>>> +       struct userfault_wait wait_mode;
> >>>>
> >>>>         /*
> >>>>          * We don't do userfault handling for the final child pid update
> >>>> @@ -466,7 +480,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> >>>>         uwq.ctx = ctx;
> >>>>         uwq.waken = false;
> >>>>
> >>>> -       blocking_state = userfaultfd_get_blocking_state(vmf->flags);
> >>>> +       wait_mode = userfaultfd_get_blocking_state(vmf->flags);
> >>>>
> >>>>          /*
> >>>>           * Take the vma lock now, in order to safely call
> >>>> @@ -488,7 +502,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> >>>>          * following the spin_unlock to happen before the list_add in
> >>>>          * __add_wait_queue.
> >>>>          */
> >>>> -       set_current_state(blocking_state);
> >>>> +       set_current_state(wait_mode.task_state);
> >>>>         spin_unlock_irq(&ctx->fault_pending_wqh.lock);
> >>>>
> >>>>         if (!is_vm_hugetlb_page(vma))
> >>>> @@ -501,7 +515,11 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> >>>>
> >>>>         if (likely(must_wait && !READ_ONCE(ctx->released))) {
> >>>>                 wake_up_poll(&ctx->fd_wqh, EPOLLIN);
> >>>> -               schedule();
> >>>> +               /* See comment in userfaultfd_get_blocking_state() */
> >>>> +               if (!wait_mode.timeout)
> >>>> +                       schedule();
> >>>> +               else
> >>>> +                       schedule_timeout(HZ / 10);
> >>>>         }
> >>>>
> >>>>         __set_current_state(TASK_RUNNING);
> >>>>
> >>>> --
> >>>> Jens Axboe
> >>> I guess the previous io_work_fault patch might have already addressed
> >>> the issue sufficiently. The later patch that adds a timeout for
> >>> userfaultfd might
> >>
> >> That one isn't guaranteed to be safe, as it's not necessarily a safe
> >> context to prune the conditions that lead to a busy loop rather than the
> >> normal "schedule until the condition is resolved". Running task_work
> >> should only be done at the outermost point in the kernel, where the task
> >> state is known sane in terms of what locks etc are being held. For some
> >> conditions the patch will work just fine, but it's not guaranteed to be
> >> the case.
> >>
> >>> not be necessary  wouldn?t returning after a timeout just cause the
> >>> same fault to repeat indefinitely again? Regardless of whether the
> >>> thread is in UN or IN state, the expected behavior should be to wait
> >>> until the page is filled or the uffd resource is released to be woken
> >>> up, which seems like the correct logic.
> >>
> >> Right, it'll just sleep timeout for a bit as not to be a 100% busy loop.
> >> That's unfortunately the best we can do for this case... The expected
> >> behavior is indeed to schedule until we get woken, however that just
> >> doesn't work if there are signals pending, or other conditions that lead
> >> to TASK_INTERRUPTIBLE + schedule() being a no-op.
> >>
> >> --
> >> Jens Axboe
> > In my testing, clearing the NOTIFY flag in the original io_work_fault
> > ensures that the next schedule correctly waits. However, adding a
>
> That's symptom fixing again - the NOTIFY flag is the thing that triggers
> for io_uring, but any legitimate signal (or task_work added with
> signaling) will cause the same issue.
>
> > timeout causes the issue to return to multiple faults again.
> > Also, after clearing the NOTIFY flag in handle_userfault,
> > it?s possible that some task work hasn?t been executed.
> > But if task_work_run isn?t added back, tasks might get lost?
> > It seems like there isn?t an appropriate place to add it back.
> > So, do you suggest adjusting the fault frequency in userfaultfd
> > to make it more rhythmic to alleviate the issue?
>
> The task_work is still there, you just removed the notification
> mechanism that tells the kernel that there's task_work there. For this
> particular case, you could re-set TIF_NOTIFY_SIGNAL at the end after
> schedule(), but again it'd only fix that specific one case, not the
> generic issue.
>
> What's the objection to the sleep approach? If the task is woken by the
> fault being filled, it'll still wake on time, no delay. If not, then it
> prevents a busy loop, which is counterproductive.
>
> --
> Jens Axboe
OK Thanks .and i’m curious about what exactly is meant by a
'specific one case 'and what qualifies as a 'generic issue' with re-set
TIF_NOTIFY_SIGNAL.
So, in your final opinion, do you think the code in io_uring is not suitable
for modification, should focus on making adjustments in userfaultfd to
mitigate the issue?

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