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

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Message-ID: <YUDkR8YtekMkuaBH@hirez.programming.kicks-ass.net>
Date:   Tue, 14 Sep 2021 20:04:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peter Oskolkov <posk@...gle.com>
Cc:     Jann Horn <jannh@...gle.com>, Peter Oskolkov <posk@...k.io>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
        Andrei Vagin <avagin@...gle.com>,
        Thierry Delisle <tdelisle@...terloo.ca>
Subject: Re: [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers

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.