Re: [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers

[Peter Oskolkov]

On Tue, Sep 14, 2021 at 11:05 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, Sep 14, 2021 at 09:29:00AM -0700, Peter Oskolkov wrote:
> > In the version of the patchset that I'm preparing to send I've decided
> > to punt on the issue and just ask the userspace to deal with locking
> > the memory as it sees fit: mlock() is available and as far as I can
>
> Sadly mlock() does not imply no faults. Someone had a too literal
> reading of the POSIX-RT spec (of which mlock is part) and figured that
> all that was required was to keep the page in memory, not avoid faults.
>
> Linux has had this bahviour for ages, PREEMPT_RT has tried to change
> this, but so far to no avail. At some point sys_mpin() was proposed to
> meet the original POSIX-RT intent, but afaict that never actually
> happened.
>
> In short, mlock() does not avoid minor faults, or even migration faults,
> which can take a fair while to resolve.

Ok, I'll go with transiently pinning pages in
__syscall_enter_from_user_work(), as you suggested. Seems easy enough
to do.

Thanks for the suggestion!