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Date:   Tue, 1 Feb 2022 18:17:34 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Michel Lespinasse <michel@lespinasse.org>,
        Linux-MM <linux-mm@kvack.org>, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, Laurent Dufour <ldufour@linux.ibm.com>,
        Jerome Glisse <jglisse@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Michal Hocko <mhocko@suse.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Matthew Wilcox <willy@infradead.org>,
        Liam Howlett <liam.howlett@oracle.com>,
        Rik van Riel <riel@surriel.com>,
        Paul McKenney <paulmck@kernel.org>,
        Song Liu <songliubraving@fb.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Minchan Kim <minchan@google.com>,
        Joel Fernandes <joelaf@google.com>,
        David Rientjes <rientjes@google.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v2 00/35] Speculative page faults
Message-ID: <YflrLpx3gpRMcu6p@linutronix.de>
References: <20220128131006.67712-1-michel@lespinasse.org>
 <20220131171434.89870a8f1ae294912e7ff19e@linux-foundation.org>
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On 2022-01-31 17:14:34 [-0800], Andrew Morton wrote:
> On Fri, 28 Jan 2022 05:09:31 -0800 Michel Lespinasse <michel@lespinasse.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