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Date:   Wed, 11 May 2022 17:22:07 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        John Dias <joaodias@google.com>,
        David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH v4] mm: fix is_pinnable_page against on cma page
Message-ID: <20220512002207.GJ1790663@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
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On Wed, May 11, 2022 at 05:12:32PM -0700, John Hubbard wrote:
> On 5/11/22 16:57, John Hubbard wrote:
> > On 5/11/22 16:45, Paul E. McKenney wrote:
> > > > 
> > > > Well no, because the "&" operation is a single operation on the CPU, and
> > > > isn't going to get split up like that.
> > > 
> > > Chiming in a bit late...
> > 
> > Much appreciated!
> > 
> > > 
> > > The usual way that this sort of thing causes trouble is if there is a
> > > single store instruction that changes the value from MIGRATE_ISOLATE
> > > to MIGRATE_CMA, and if the compiler decides to fetch twice, AND twice,
> > 
> > Doing an AND twice for "x & constant" this definitely blows my mind. Is
> > nothing sacred? :)
> > 
> > > and then combine the results.? This could give a zero outcome where the
> > > underlying variable never had the value zero.
> > > 
> > > Is this sort of thing low probability?
> > > 
> > > Definitely.
> > > 
> > > Isn't this sort of thing prohibited?
> > > 
> > > Definitely not.
> > > 
> > > So what you have will likely work for at least a while longer, but it
> > > is not guaranteed and it forces you to think a lot harder about what
> > > the current implementations of the compiler can and cannot do to you.
> > > 
> > > The following LWN article goes through some of the possible optimizations
> > > (vandalisms?) in this area: https://lwn.net/Articles/793253/
> > > 
> > 
> > hmm, I don't think we hit any of those? cases, do we? Because here, the
> > "write" side is via a non-inline function that I just don't believe the
> > compiler is allowed to call twice. Or is it?
> > 
> > Minchan's earlier summary:
> > 
> > CPU 0???????????????????????? CPU1
> > 
> > 
> >  ????????????????????????????? set_pageblock_migratetype(MIGRATE_ISOLATE)
> > 
> > if (get_pageblock_migrate(page) & MIGRATE_CMA)
> > 
> >  ????????????????????????????? set_pageblock_migratetype(MIGRATE_CMA)
> > 
> > if (get_pageblock_migrate(page) & MIGRATE_ISOLATE)
> > 
> > ...where set_pageblock_migratetype() is not inline.
> > 
> > thanks,
> 
> Let me try to say this more clearly: I don't think that the following
> __READ_ONCE() statement can actually help anything, given that
> get_pageblock_migratetype() is non-inlined:
> 
> +	int __mt = get_pageblock_migratetype(page);
> +	int mt = __READ_ONCE(__mt);
> +
> +	if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE))
> +		return false;
> 
> 
> Am I missing anything here?

In the absence of future aggression from link-time optimizations (LTO),
you are missing nothing.

							Thanx, Paul