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

[Jerome Glisse <jglisse@redhat.com>]

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
>