Re: PROBLEM: userfaultfd REGISTER minor mode on MAP_PRIVATE range fails

[Peter Xu <peterx@redhat.com>]

On Mon, Sep 29, 2025 at 03:44:52PM -0400, David P. Reed wrote:
> I thought it was a general purpose interface. My mistake. But I think it
> can be more general, at least encompassing my goal of having a userspace
> "interface" that monitors processes' page faults.

To James: thanks for the great writeup.  Somehow, I just feel like userfaultfd
(as a linux submodule) got some sheer luck to have you around. :)

To David: just to say, I still think it's a general purpose interface, at
least that's the hope..

I agree with you at least on one point you mentioned, that shmem also can
swap, and that was accounted as minor faults when swapin happens at a
specific virtual address.  It doesn't sound fair if anon isn't doing the
same. Indeed.

It was just not in the radar when minor fault was introduced by Axel, even
it was for a solo purpose for live migration at that time.. but the hope is
the interface designed should service a generic purpose.

Now the problem is, userfaultfd wasn't initially used for monitoring system
activities.  As its name implies, it provides the userspace a way to
resolve a fault, but only if a fault happens first..

Meanwhile, system activities should definitely at least involve swapouts,
which unfortunately doesn't involve page faults, but only happen the other
way round when the system wants to secretly move things out.. that is what
userfaultfd is out of control.

It just sounds like it won't suffice your need even if we could add minor
fault support for anon private memories on swap cache. However, if
userfaultfd is used to do everything (including swap in/outs), then it's by
nature all trappable + accountable, on both swap in/outs to/from any media.
Then swapout will be driven by the userspace too, then everything will be
in solid control, including monitoring of the activities.

Thanks,

-- 
Peter Xu