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References: <20190513003819.356-1-hsinyi@chromium.org> <20190513003819.356-2-hsinyi@chromium.org> <20190513085853.GB9271@rapoport-lnx> <CAJMQK-hKrU2J0_uGe3eO_JTNwM=HRkXbDx2u45izcdD7wqwGeQ@mail.gmail.com>
From:   Stephen Boyd <swboyd@chromium.org>
Subject: Re: [PATCH v2 2/2] amr64: map FDT as RW for early_init_dt_scan()
Cc:     "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@lists.infradead.org>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Frank Rowand <frowand.list@gmail.com>,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        Kees Cook <keescook@chromium.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Architecture Mailman List <boot-architecture@lists.linaro.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Miles Chen <miles.chen@mediatek.com>,
        James Morse <james.morse@arm.com>,
        Andrew Murray <andrew.murray@arm.com>
To:     Hsin-Yi Wang <hsinyi@chromium.org>,
        Mike Rapoport <rppt@linux.ibm.com>
Message-ID: <155786794318.14659.2925897827978978040@swboyd.mtv.corp.google.com>
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Date:   Tue, 14 May 2019 14:05:43 -0700
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Quoting Hsin-Yi Wang (2019-05-13 04:14:32)
> On Mon, May 13, 2019 at 4:59 PM Mike Rapoport <rppt@linux.ibm.com> wrote:
>=20
> >
> > This makes the fdt mapped without the call to meblock_reserve(fdt) which
> > makes the fdt memory available for memblock allocations.
> >
> > Chances that is will be actually allocated are small, but you know, thi=
ngs
> > happen.
> >
> > IMHO, instead of calling directly __fixmap_remap_fdt() it would be bett=
er
> > to add pgprot parameter to fixmap_remap_fdt(). Then here and in kaslr.c=
 it
> > can be called with PAGE_KERNEL and below with PAGE_KERNEL_RO.
> >
> > There is no problem to call memblock_reserve() for the same area twice,
> > it's essentially a NOP.
> >
> Thanks for the suggestion. Will update fixmap_remap_fdt() in next patch.
>=20
> However, I tested on some arm64 platform, if we also call
> memblock_reserve() in kaslr.c, would cause warning[1] when
> memblock_reserve() is called again in setup_machine_fdt(). The warning
> comes from https://elixir.bootlin.com/linux/latest/source/mm/memblock.c#L=
601
> ```
> if (type->regions[0].size =3D=3D 0) {
>   WARN_ON(type->cnt !=3D 1 || type->total_size);
>   ...
> ```
>=20
> Call memblock_reserve() multiple times after setup_machine_fdt()
> doesn't have such warning though.
>=20
> I didn't trace the real reason causing this. But in this case, maybe
> don't call memblock_reserve() in kaslr?
>=20

Why not just have fixmap_remap_fdt() that maps it as RW and reserves
memblock once, and then call __fixmap_remap_fdt() with RO after
early_init_dt_scan() or unflatten_device_tree() is called? Why the
desire to call memblock_reserve() twice or even three times?