linux-kernel - Re: Userspace notifications for observing userfaultfd faults

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Message-ID: <CAOp6jLaSLC4O0ZayFz1BiO6UqC7pK_umFUb29bgXDu6U5dUQgg@mail.gmail.com>
Date:   Wed, 12 May 2021 10:15:21 +1200
From:   "Robert O'Callahan" <roc@...nos.co>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>, Peter Xu <peterx@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, Kyle Huey <khuey@...nos.co>
Subject: Re: Userspace notifications for observing userfaultfd faults

On Wed, May 12, 2021 at 6:12 AM Axel Rasmussen <axelrasmussen@...gle.com> wrote:
> Is some combination of bpf and kprobes a possible solution? There are
> some seemingly relevant examples here:
> https://github.com/iovisor/bpftrace/blob/master/docs/tutorial_one_liners.md
>
> I haven't tried it, but it seems like attaching to handle_userfault()
> would give similar information to perf_count_sw_page_faults, but for
> userfaults.

That would probably work in some cases, but as Kyle said that requires
privileges and currently rr can run unprivileged (if you set
perf_event_paranoid to 1 or less) and usually does. Also, AFAIK,
kprobing handle_userfault would not be a stable ABI.

Rob