Re: userfaultfd: usability issue due to lack of UFFD events ordering

From: Peter Xu <peterx@redhat.com>
To: Nadav Amit <nadav.amit@gmail.com>
Cc: Mike Rapoport <rppt@kernel.org>,
	David Hildenbrand <david@redhat.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Linux-MM <linux-mm@kvack.org>
Subject: Re: userfaultfd: usability issue due to lack of UFFD events ordering
Date: Thu, 10 Feb 2022 15:48:30 +0800	[thread overview]
Message-ID: <YgTDTjrhoiyH4ZTr@xz-m1.local> (raw)
In-Reply-To: <B2B2DFF0-7967-4F80-8AAC-3DB0B3911CED@gmail.com>

Hi, Nadav & all,

On Mon, Jan 31, 2022 at 02:39:01PM -0800, Nadav Amit wrote:
> There are use-cases in which you do need to know the order between
> user-initiated MADV_DONTNEED and page-faults. For instance, if you
> build a userspace paging mechanism, you need to know whether the
> page content is zero or whatever is held in the disk.

When there's no uffd monitor, concurrent page faults with MADV_DONTNEED can
already result in undefined behavior, right? Assuming the page fault is a write
with non-zero data, then the page can either contain zero or non-zero data
at last, IIUC.

If above is true, I'm wondering whether it's already impossible to do it right
when there is an uffd monitor?

Thanks,

-- 
Peter Xu