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From: Peter Xu <[email protected]>
To: Nadav Amit <[email protected]>
Cc: [email protected], Nadav Amit <[email protected]>,
	Jens Axboe <[email protected]>,
	Andrea Arcangeli <[email protected]>,
	Alexander Viro <[email protected]>,
	[email protected], [email protected],
	[email protected]
Subject: Re: [RFC PATCH 03/13] selftests/vm/userfaultfd: wake after copy failure
Date: Mon, 21 Dec 2020 14:28:46 -0500	[thread overview]
Message-ID: <20201221192846.GH6640@xz-x1> (raw)
In-Reply-To: <[email protected]>

On Sat, Nov 28, 2020 at 04:45:38PM -0800, Nadav Amit wrote:
> From: Nadav Amit <[email protected]>
> 
> When userfaultfd copy-ioctl fails since the PTE already exists, an
> -EEXIST error is returned and the faulting thread is not woken. The
> current userfaultfd test does not wake the faulting thread in such case.
> The assumption is presumably that another thread set the PTE through
> copy/wp ioctl and would wake the faulting thread or that alternatively
> the fault handler would realize there is no need to "must_wait" and
> continue. This is not necessarily true.
> 
> There is an assumption that the "must_wait" tests in handle_userfault()
> are sufficient to provide definitive answer whether the offending PTE is
> populated or not. However, userfaultfd_must_wait() test is lockless.
> Consequently, concurrent calls to ptep_modify_prot_start(), for
> instance, can clear the PTE and can cause userfaultfd_must_wait()
> to wrongly assume it is not populated and a wait is needed.

Yes userfaultfd_must_wait() is lockless, however my understanding is that we'll
enqueue before reading the page table, which seems to me that we'll always get
notified even the race happens.  Should apply to either UFFDIO_WRITEPROTECT or
UFFDIO_COPY, iiuc, as long as we follow the order of (1) modify pgtable (2)
wake sleeping threads.  Then it also means that when must_wait() returned true,
it should always get waked up when fault resolved.

Taking UFFDIO_COPY as example, even if UFFDIO_COPY happen right before
must_wait() calls:

       worker thread                       uffd thread
       -------------                       -----------

   handle_userfault
    spin_lock(fault_pending_wqh)
    enqueue()
    set_current_state(INTERRUPTIBLE)
    spin_unlock(fault_pending_wqh)
    must_wait()
      lockless walk page table
                                           UFFDIO_COPY
                                             fill in the hole
                                             wake up threads
                                               (this will wake up worker thread too?)
    schedule()
      (which may return immediately?)

While here fault_pending_wqh is lock protected. I just feel like there's some
other reason to cause the thread to stall.  Or did I miss something?

Thanks,

-- 
Peter Xu


  reply	other threads:[~2020-12-21 19:30 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-29  0:45 [RFC PATCH 00/13] fs/userfaultfd: support iouring and polling Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 01/13] fs/userfaultfd: fix wrong error code on WP & !VM_MAYWRITE Nadav Amit
2020-12-01 21:22   ` Mike Kravetz
2020-12-21 19:01     ` Peter Xu
2020-11-29  0:45 ` [RFC PATCH 02/13] fs/userfaultfd: fix wrong file usage with iouring Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 03/13] selftests/vm/userfaultfd: wake after copy failure Nadav Amit
2020-12-21 19:28   ` Peter Xu [this message]
2020-12-21 19:51     ` Nadav Amit
2020-12-21 20:52       ` Peter Xu
2020-12-21 20:54         ` Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 04/13] fs/userfaultfd: simplify locks in userfaultfd_ctx_read Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 05/13] fs/userfaultfd: introduce UFFD_FEATURE_POLL Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 06/13] iov_iter: support atomic copy_page_from_iter_iovec() Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 07/13] fs/userfaultfd: support read_iter to use io_uring Nadav Amit
2020-11-30 18:20   ` Jens Axboe
2020-11-30 19:23     ` Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 08/13] fs/userfaultfd: complete reads asynchronously Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 09/13] fs/userfaultfd: use iov_iter for copy/zero Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 10/13] fs/userfaultfd: add write_iter() interface Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 11/13] fs/userfaultfd: complete write asynchronously Nadav Amit
2020-12-02  7:12   ` Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 12/13] fs/userfaultfd: kmem-cache for wait-queue objects Nadav Amit
2020-11-30 19:51   ` Nadav Amit
2020-11-29  0:45 ` [RFC PATCH 13/13] selftests/vm/userfaultfd: iouring and polling tests Nadav Amit

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