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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: i386: CONFIG_HIGHMEM4G only detects 3 GB out of 4 GB of memory
Date:   Sat, 10 Oct 2020 15:26:32 -0700
Message-Id: <BC8FECF1-7703-435A-BA60-F0B45D3CC940@amacapital.net>
References: <321879b2-d7f4-b1f7-8f5b-f757500e43a8@molgen.mpg.de>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, LKML <linux-kernel@vger.kernel.org>
In-Reply-To: <321879b2-d7f4-b1f7-8f5b-f757500e43a8@molgen.mpg.de>
To:     Paul Menzel <pmenzel@molgen.mpg.de>
Precedence: bulk


> On Oct 10, 2020, at 2:50 PM, Paul Menzel <pmenzel@molgen.mpg.de> wrote:
>=20
> =EF=BB=BFDear Linux folks,
>=20
>=20
> On an Asus F2A85-M PRO with two 2 GB RAM modules installed, and an APU dev=
ice, building Linux with `ARCH=3Di386` and `CONFIG_HIGHMEM4G=3Dy` only 3 GB s=
eem to be detected: 2.2 GB according to `free -h` plus the 768 MB for APU gr=
aphics memory).
>=20
>> [    0.065059] Memory: 2285148K/2324512K available (11785K kernel code, 8=
92K rwdata, 2748K rodata, 668K init, 544K bss, 39364K reserved, 0K cma-reser=
ved, 1423796K highmem)
>=20
>> [    0.402082] calling  populate_rootfs+0x0/0xa1 @ 1
>=20
>=20
>> $ free -h
>>              total        used        free      shared  buff/cache   avai=
lable
>> Mem:          2,2Gi        72Mi       2,0Gi        13Mi       130Mi      =
 2,0Gi
>> Swap:            0B          0B          0B
>=20
> With `CONFIG_HIGHMEM64G=3Dy` the whole 4 GB are used (3.1 GB + 768 MB for A=
PU graphics memory).
>=20
>> [    0.121036] Memory: 3229952K/3356700K available (10301K kernel code, 8=
21K rwdata, 2700K rodata, 708K init, 540K bss, 126748K reserved, 0K cma-rese=
rved, 2449840K highmem)
>=20
>> [    0.450668] calling  populate_rootfs+0x0/0xa1 @ 1
>=20
> The Kconfig help text for `HIGHMEM4G` says:
>=20
>> Select this if you have a 32-bit processor and between 1 and 4           =
                                                                     =E2=94=82=
   gigabytes of physical RAM.
>=20
> As I only have 4 GB, I chose that to save 50 ms (maybe only due to less me=
mory detected), and thought non-PAE kernels can use 4 GB of memory.
>=20

Your memory map contains:

BIOS-e820: [mem 0x0000000100001000-0x000000013effffff] usable

That=E2=80=99s 0x3effffff bytes mapped at a physical address above 4G.

The 4G limit without PAE isn=E2=80=99t, strictly speaking, about how much RA=
M can be used; it=E2=80=99s about the maximum usable physical address.  One m=
ight wonder why your firmware set up your memory map like this.

But there=E2=80=99s a much bigger issue: why on Earth are you running a 32-b=
it kernel on a relatively new machine like this?=