linux-kernel - Re: [PATCH 3/7] Add a UFFD_SECURE flag to the userfaultfd API.

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Date:   Wed, 23 Oct 2019 20:23:12 -0400
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Nick Kralevich <nnk@...gle.com>,
        Nosh Minwalla <nosh@...gle.com>,
        Tim Murray <timmurray@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Linux API <linux-api@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/7] Add a UFFD_SECURE flag to the userfaultfd API.

On Wed, Oct 23, 2019 at 01:05:47PM -0700, Daniel Colascione wrote:
> This is a debate that won't get resolved here. A ton of work has gone
> into namespaces, migration, various cgroup things, and so on, and I
> don't see that work getting torn out.

This is precisely why I thought it was a good idea to support the
non-cooperative use case too even though we had no immediate use for
it.

> Sure they can. Can't we stick processes in a memcg and set a
> memory.high threshold beyond which threads in that cgroup will enter
> direct reclaim on page allocations? I'd call that throttling.

The uffd-wp solution during the throttling can resolve a wrprotect
fault in the parent for every 4k page that has been written to disk
and it'll prioritize writing to disk those userfaults that are
currently blocked. I don't see how you could reach an equivalent
optimal runtime without uffd-wp and just with memcg because the
snapshot process won't have a clue which pages are been duped by the
COWs. The uffd-wp by avoding fork will also avoid more expensive MM
switches during the snapshot.

> This issue *has* to get fixed one way or another.

Sure.