Message-ID: <CANHzP_vws+MPqt4u3pvWebM_ZObz_FZ2P2Zz20fPpWon8Kxxhg@mail.gmail.com>
Date: Thu, 24 Apr 2025 23:51:42 +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 23:21写道：
>
> On 4/24/25 9:12 AM, ??? wrote:
> > Jens Axboe <axboe@...nel.dk> ?2025?4?24??? 22:53???
> >>
> >> 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.
>
> I already outlined that in earlier replies, find the email that states
> the various conditions that can lead to schedule() w/TASK_INTERRUPTIBLE
> to return immediately rather than sleeping. TIF_NOTIFY_SIGNAL is _one_
> such condition, it's not _all_ conditions.
>
> > 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?
>
> The problem isn't in io_uring in the first place, you just happened to
> trip over it via that path. I even sent out a test case that
> demonstrates how to trigger this without io_uring as well. I'm a bit
> puzzled as to why all of this isn't clear already.
>
> --
> Jens Axboe
Thank you. I think the final solution for my scenario might involve the
user-space monitoring of the uffd business thread to ensure that it can
unregister the uffd memory, allowing the fault process to exit the loop
in IOU via an error return, which would further help exit the VM
process's D state.

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