linux-kernel - Re: [PATCH v2 33/35] arm64/mm: attempt speculative mm faults first

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Date:   Mon, 7 Feb 2022 09:39:19 -0800
From:   Michel Lespinasse <michel@...pinasse.org>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Michel Lespinasse <michel@...pinasse.org>,
        Linux-MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.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>
Subject: Re: [PATCH v2 33/35] arm64/mm: attempt speculative mm faults first

On Tue, Feb 01, 2022 at 10:58:03AM +0200, Mike Rapoport wrote:
> On Mon, Jan 31, 2022 at 12:07:29AM -0800, Michel Lespinasse wrote:
> > On Sun, Jan 30, 2022 at 11:13:26AM +0200, Mike Rapoport wrote:
> > > The speculative page fault implementation here (and for PowerPC as well)
> > > looks very similar to x86. Can we factor it our rather than copy 3 (or
> > > more) times?
> > 
> > In each arch, the speculative code was written along the lines of the
> > existing non-speculative code, so that behavior would be unchanged
> > when speculation succeeds.
> > 
> > Now each arch's existing, non-speculative code paths are quite similar,
> > but they do have small differences as to how they implement various
> > permission checks, protection keys and the like. The same small
> > differences end up being reflected in the new speculative code paths.
> > 
> > I agree it would be nice if this code could be unified between archs,
> > but IMO this should start with the existing non-speculative code -
> > I don't think it would make sense to try unifying the new speculative
> > code while trying to follow the behavior of the non-unified old
> > non-speculative code paths...
> 
> Then maybe this unification can be done as the ground work for the
> speculative page fault handling?

I feel like this is quite unrelated, and that introducing such
artificial dependencies is a bad work habit we have here in linux MM...

That said, unifying the PF code between archs would be an interesting
project on its own. The way I see it, there could be a unified page
fault handler, with some arch specific parts defined as inline
functions.  I can see myself making an x86/arm64/powerpc initial
proposal if there is enough interest for it, but I'm not sure how
extending it to more exotic archs would go - I think this would have
to involve arch maintainers at least for testing purposes, and I'm not
sure if they'd have any bandwidth for such a project...

--
Michel "walken" Lespinasse