Re: [PATCH v2 03/26] userfaultfd: don't retake mmap_sem to emulate NOPAGE

Jerome Glisse
2019-02-21T15:34:19Z

On Tue, Feb 12, 2019 at 10:56:09AM +0800, Peter Xu wrote:
> The idea comes from the upstream discussion between Linus and Andrea:
>
> https://lkml.org/lkml/2017/10/30/560
>
> A summary to the issue: there was a special path in handle_userfault()
> in the past that we'll return a VM_FAULT_NOPAGE when we detected
> non-fatal signals when waiting for userfault handling. We did that by
> reacquiring the mmap_sem before returning. However that brings a risk
> in that the vmas might have changed when we retake the mmap_sem and
> even we could be holding an invalid vma structure.
>
> This patch removes the risk path in handle_userfault() then we will be
> sure that the callers of handle_mm_fault() will know that the VMAs
> might have changed. Meanwhile with previous patch we don't lose
> responsiveness as well since the core mm code now can handle the
> nonfatal userspace signals quickly even if we return VM_FAULT_RETRY.
>
> Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Jérôme Glisse <jglisse@redhat.com>

> ---
> fs/userfaultfd.c | 24 ------------------------
> 1 file changed, 24 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 89800fc7dc9d..b397bc3b954d 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -514,30 +514,6 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>
> __set_current_state(TASK_RUNNING);
>
> - if (return_to_userland) {
> - if (signal_pending(current) &&
> - !fatal_signal_pending(current)) {
> - /*
> - * If we got a SIGSTOP or SIGCONT and this is
> - * a normal userland page fault, just let
> - * userland return so the signal will be
> - * handled and gdb debugging works. The page
> - * fault code immediately after we return from
> - * this function is going to release the
> - * mmap_sem and it's not depending on it
> - * (unlike gup would if we were not to return
> - * VM_FAULT_RETRY).
> - *
> - * If a fatal signal is pending we still take
> - * the streamlined VM_FAULT_RETRY failure path
> - * and there's no need to retake the mmap_sem
> - * in such case.
> - */
> - down_read(&mm->mmap_sem);
> - ret = VM_FAULT_NOPAGE;
> - }
> - }
> -
> /*
> * Here we race with the list_del; list_add in
> * userfaultfd_ctx_read(), however because we don't ever run
> --
> 2.17.1
>