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Date:   Wed, 15 Sep 2021 21:10:23 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Jann Horn <jannh@google.com>, Peter Oskolkov <posk@google.com>,
        Peter Oskolkov <posk@posk.io>, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Paul Turner <pjt@google.com>, Ben Segall <bsegall@google.com>,
        Andrei Vagin <avagin@google.com>,
        Thierry Delisle <tdelisle@uwaterloo.ca>
Subject: Re: [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers
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On Wed, Sep 15, 2021 at 09:50:41AM -0700, Andy Lutomirski wrote:
> What's the precise problem?  The code would be roughly:
> 
> if (try_pin_the_page) {
>   write it;
>   unpin;
> } else {
>   do nothing -- .fault will fill in the correct contents.
> }
> 
> The time this takes is nondeterministic, but it's bounded and short.

You can't do this from sched_submit_work(), you might already hold
mmap_sem. Hence the suggestion to, for UMCG registered threads, pin the
page(s) on syscall-enter and unpin on either schedule() or
syscall-return.