linux-kernel - Re: [PATCH v2 00/35] Speculative page faults

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Message-ID: <20220207173953.GD12302@lespinasse.org>
Date:   Mon, 7 Feb 2022 09:39:53 -0800
From:   Michel Lespinasse <michel@...pinasse.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michel Lespinasse <michel@...pinasse.org>,
        Linux-MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org,
        kernel-team@...com, Laurent Dufour <ldufour@...ux.ibm.com>,
        Jerome Glisse <jglisse@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Davidlohr Bueso <dave@...olabs.net>,
        Liam Howlett <liam.howlett@...cle.com>,
        Rik van Riel <riel@...riel.com>,
        Paul McKenney <paulmck@...nel.org>,
        Song Liu <songliubraving@...com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Minchan Kim <minchan@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH v2 00/35] Speculative page faults

On Tue, Feb 01, 2022 at 02:20:39AM +0000, Matthew Wilcox wrote:
> On Mon, Jan 31, 2022 at 05:14:34PM -0800, Andrew Morton wrote:
> > On Fri, 28 Jan 2022 05:09:31 -0800 Michel Lespinasse <michel@...pinasse.org> wrote:
> > > The next step is to walk down the existing page table tree to find the
> > > current pte entry. This is done with interrupts disabled to avoid
> > > races with munmap().
> > 
> > Sebastian, could you please comment on this from the CONFIG_PREEMPT_RT
> > point of view?
> 
> I am not a fan of this approach.  For other reasons, I think we want to
> switch to RCU-freed page tables, and then we can walk the page tables
> with the RCU lock held.  Some architectures already RCU-free the page
> tables, so I think it's just a matter of converting the rest.

Note - I have no problem with switching to RCU-freed page tables
everywhere when and if we end up needing to. I just don't see that
this need comes from the SPF patchset, so I don't think this should
be introduced as an artificial dependency.

--
Michel "walken" Lespinasse