linux-kernel - Re: [PATCH v2 7/9] mm: Calc the right pfn if page size is not 4K

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Message-ID: <CAMj1kXFkso9i8OqHOA_XMu90mrSsSqR1uSmcUTxnD0SxStuTpw@mail.gmail.com>
Date:   Thu, 21 Apr 2022 00:39:32 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        Wupeng Ma <mawupeng1@...wei.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, hpa@...cr.com,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Mike Rapoport <rppt@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Joerg Roedel <jroedel@...e.de>, songmuchun@...edance.com,
        macro@...am.me.uk, Frederic Weisbecker <frederic@...nel.org>,
        W_Armin@....de, John Garry <john.garry@...wei.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        chenhuacai@...nel.org, David Hildenbrand <david@...hat.com>,
        gpiccoli@...lia.com, Mark Rutland <mark.rutland@....com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        linux-ia64@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH v2 7/9] mm: Calc the right pfn if page size is not 4K

On Thu, 21 Apr 2022 at 00:27, Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Tue, 19 Apr 2022 20:29:27 +0200 Ard Biesheuvel <ardb@...nel.org> wrote:
>
> > > > --- a/mm/page_alloc.c
> > > > +++ b/mm/page_alloc.c
> > > > @@ -7870,7 +7870,7 @@ static void __init find_zone_movable_pfns_for_nodes(void)
> > > >
> > > >                       usable_startpfn = memblock_region_memory_base_pfn(r);
> > > >
> > > > -                     if (usable_startpfn < 0x100000) {
> > > > +                     if (usable_startpfn < PHYS_PFN(SZ_4G)) {
> > > >                               mem_below_4gb_not_mirrored = true;
> > > >                               continue;
> > > >                       }
> > >
> > > Regardless PFN value should never be encoded directly.
> > >
> > > Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>
> >
> > Acked-by: Ard Biesheuvel <ardb@...nel.org>
> >
> > Andrew, can you please take this one through the -mm tree? The rest of
> > the series needs a bit more work, but is an obvious fix and there is
> > no point in holding it up.
>
> Sure.
>
> I'm not seeing any description of the runtime effects of this
> shortcoming.  I tentatively queued the fix for 5.18, without a
> cc:stable for backporting.  But that might not be the best decision?
>

As far as I can tell, mirrored memory is only used on x86 today, where
pages are always 4k.

However, the whole notion of memory below 4 GB being special is a
x86-ism, and so this logic does not appear to extrapolate to other
architectures anyway, and probably needs more work.

So definitely not a backportable fix, but just an incremental
improvement, so either 5.18 or 5.19 should be fine afaict (and no
cc:stable)