linux-kernel - i386: CONFIG_HIGHMEM4G only detects 3 GB out of 4 GB of memory

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Date:   Sat, 10 Oct 2020 23:50:47 +0200
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: i386: CONFIG_HIGHMEM4G only detects 3 GB out of 4 GB of memory

Dear Linux folks,


On an Asus F2A85-M PRO with two 2 GB RAM modules installed, and an APU 
device, building Linux with `ARCH=i386` and `CONFIG_HIGHMEM4G=y` only 3 
GB seem to be detected: 2.2 GB according to `free -h` plus the 768 MB 
for APU graphics memory).

> [    0.065059] Memory: 2285148K/2324512K available (11785K kernel code, 892K rwdata, 2748K rodata, 668K init, 544K bss, 39364K reserved, 0K cma-reserved, 1423796K highmem)

> [    0.402082] calling  populate_rootfs+0x0/0xa1 @ 1


> $ free -h
>               total        used        free      shared  buff/cache   available
> Mem:          2,2Gi        72Mi       2,0Gi        13Mi       130Mi       2,0Gi
> Swap:            0B          0B          0B

With `CONFIG_HIGHMEM64G=y` the whole 4 GB are used (3.1 GB + 768 MB for 
APU graphics memory).

> [    0.121036] Memory: 3229952K/3356700K available (10301K kernel code, 821K rwdata, 2700K rodata, 708K init, 540K bss, 126748K reserved, 0K cma-reserved, 2449840K highmem)

> [    0.450668] calling  populate_rootfs+0x0/0xa1 @ 1

The Kconfig help text for `HIGHMEM4G` says:

> Select this if you have a 32-bit processor and between 1 and 4                                                                                │   
> gigabytes of physical RAM.

As I only have 4 GB, I chose that to save 50 ms (maybe only due to less 
memory detected), and thought non-PAE kernels can use 4 GB of memory.


Kind regards,

Paul

View attachment "linux-5.9-rc8+-dmesg-highmem4.txt" of type "text/plain" (136556 bytes)

View attachment "linux-5.9-rc8+-dmesg-highmem64-pae.txt" of type "text/plain" (138546 bytes)