Re: Using userfaultfd with KVM's async page fault handling causes processes to hung waiting for mmap_lock to be released

From: Peter Xu <peterx@redhat.com>
To: Hillf Danton <hdanton@sina.com>
Cc: Dimitris Siakavaras <jimsiak@cslab.ece.ntua.gr>,
	Axel Rasmussen <axelrasmussen@google.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: Using userfaultfd with KVM's async page fault handling causes processes to hung waiting for mmap_lock to be released
Date: Thu, 20 Jul 2023 16:07:56 -0400	[thread overview]
Message-ID: <ZLmUHI+iyhj6QcYL@x1n> (raw)
In-Reply-To: <20230720103534.312-1-hdanton@sina.com>

On Thu, Jul 20, 2023 at 06:35:34PM +0800, Hillf Danton wrote:
> On Wed, Jul 19, 2023 at 2:16 PM Axel Rasmussen <axelrasmussen@google.com> wrote:
> >
> > I think perhaps the right thing to do is to have handle_userfault() release
> > mmap_lock when it returns VM_FAULT_NOPAGE, and to have GUP deal with that
> > appropriately? But, some investigation is required to be sure that's okay to do
> > in the other non-GUP ways we can end up in handle_userfault().
> 
> See if making kworker special works.
> 
> --- x/fs/userfaultfd.c
> +++ y/fs/userfaultfd.c
> @@ -457,6 +457,8 @@ vm_fault_t handle_userfault(struct vm_fa
>  		 * close the uffd.
>  		 */
>  		ret = VM_FAULT_NOPAGE;
> +		if (current->flags & PF_WQ_WORKER)
> +			ret = VM_FAULT_OOM;
>  		goto out;
>  	}

Sorry this won't work - we need userfault to work with all forms of
kworkers, especially including kvm async pf. Thanks.

-- 
Peter Xu