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Thu, 24 Apr 2025 08:21:19 -0700 (PDT) Message-ID: Date: Thu, 24 Apr 2025 09:21:18 -0600 Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] io_uring: Add new functions to handle user fault scenarios To: =?UTF-8?B?5aec5pm65Lyf?= Cc: viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, akpm@linux-foundation.org, peterx@redhat.com, asml.silence@gmail.com, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, io-uring@vger.kernel.org References: <20250422162913.1242057-1-qq282012236@gmail.com> <20250422162913.1242057-2-qq282012236@gmail.com> <14195206-47b1-4483-996d-3315aa7c33aa@kernel.dk> <7bea9c74-7551-4312-bece-86c4ad5c982f@kernel.dk> <52d55891-36e3-43e7-9726-a2cd113f5327@kernel.dk> <5c20b5ca-ce41-43c4-870a-c50206ab058d@kernel.dk> <1ed67bb5-5d3d-4af8-b5a7-4f644186708b@kernel.dk> From: Jens Axboe Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/24/25 9:12 AM, ??? wrote: > Jens Axboe ?2025?4?24??? 22:53??? >> >> On 4/24/25 8:45 AM, ??? wrote: >>> Jens Axboe ?2025?4?24??? 22:13??? >>>> >>>> On 4/24/25 8:08 AM, ??? wrote: >>>>> Jens Axboe ?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