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

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Date:   Tue, 1 Feb 2022 18:17:34 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     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>,
        Matthew Wilcox <willy@...radead.org>,
        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>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 00/35] Speculative page faults

On 2022-01-31 17:14:34 [-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 applied the series on top of RT and gave it shot. Nothing out of the
ordinary happened so that is good.

>From browsing through the code:
- speculative_page_walk_begin() seems to disable interrupts.
  There is a spin_trylock() invocation in that area. That is okay since
  it is never invoked from in_IRQ(). But there should not be any regular
  spin_lock() in such a section.

- We do have a seqcount API. So instead of mmap_seq_read_start() one
  could use raw_read_seqcount(). The lockdep bits would also check if
  the associated lock (in this case mmap_lock) is held in the write
  path.

- The read side (mmap_seq_read_start()) does not attempt to stabilize
  the counter (waiting for even) which is good. Otherwise special care
  would be needed ;)

Sebastian