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

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Message-ID: <YUDmwsf8Lf7HQ3+s@hirez.programming.kicks-ass.net>
Date:   Tue, 14 Sep 2021 20:15:30 +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 08:04:55PM +0200, Peter Zijlstra 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.

Also, even if it did, that would still not be acceptible because
userspace could fail to call mlock() at which point mis-behaving
userspace can deadlock the kernel, which is a no-no.