Hi Arnd,

On Thu, Feb 13, 2020 at 5:54 PM Arnd Bergmann [off-list ref] wrote:

On Wed, Feb 12, 2020 at 9:50 AM Russell King - ARM Linux admin
[off-list ref] wrote:

quoted

On Tue, Feb 11, 2020 at 05:03:02PM -0800, Linus Torvalds wrote:

quoted

So at least my gut feel is that the arm people don't have any big
reason to push for maintaining HIGHMEM support either.

But I'm adding a couple of arm people and the arm list just in case
they have some input.

[ Obvious background for newly added people: we're talking about
making CONFIG_HIGHMEM a deprecated feature and saying that if you want
to run with lots of memory on a 32-bit kernel, you're doing legacy
stuff and can use a legacy kernel ]

Well, the recent 32-bit ARM systems generally have more than 1G
of memory, so make use of highmem as a rule.  You're probably
talking about crippling support for any 32-bit ARM system produced
in the last 8 to 10 years.

What I'm observing in the newly added board support is that memory
configurations are actually going down, driven by component cost.
512MB is really cheap (~$4) these days with a single 256Mx16 DDR3
chip or two 128Mx16. Going beyond 1GB is where things get expensive
with either 4+ chips or LPDDR3/LPDDR4 memory.

For designs with 1GB, we're probably better off just using
CONFIG_VMSPLIT_3G_OPT (without LPAE) anyway, completely
avoiding highmem. That is particularly true on systems with a custom
kernel configuration.

2GB machines are less common, but are definitely important, e.g.
MT6580 based Android phones and some industrial embedded machines
that will live a long time. I've recently seen reports of odd behavior
with CONFIG_VMSPLIT_2G and plus CONFIG_HIGHMEM and a 7:1
ratio of lowmem to highmem that apparently causes OOM despite lots
of lowmem being free. I suspect a lot of those workloads would still be
better off with a CONFIG_VMSPLIT_2G_OPT (1.75 GB user, 2GB
linear map). That config unfortunately has a few problems, too:
- nobody has implemented it
- it won't work with LPAE and therefore cannot support hardware
  that relies on high physical addresses for RAM or MMIO
  (those could run CONFIG_VMSPLIT_2G at the cost of wasting
  12.5% of RAM).
- any workload that requires the full 3GB of virtual address space won't
  work at all. This might be e.g. MAP_FIXED users, or build servers
  linking large binaries.
It will take a while to find out what kinds of workloads suffer the most
from a different vmsplit and what can be done to address that, but we
could start by changing the kernel defconfig and distro builds to see
who complains ;-)

I think 32-bit ARM machines with 3GB or more are getting very rare,
but some still exist:
- The Armada XP development board had a DIMM slot that could take
  large memory (possibly up to 8GB with LPAE). This never shipped as
  a commercial product, but distro build servers sometimes still run on
  this, or on the old Calxeda or Keystone server systems.
- a few early i.MX6 boards  (e.g. HummingBoard) came had 4GB of
  RAM, though none of these seem to be available any more.
- High-end phones from 2013/2014 had 3GB LPDDR3 before getting
  obsoleted by 64-bit phones. Presumably none of these ever ran
  Linux-4.x or newer.
- My main laptop is a RK3288 based Chromebook with 4GB that just
  got updated to linux-4.19 by Google. Official updates apparently
  stop this summer, but it could easily run Debian later on.
- Some people run 32-bit kernels on a 64-bit Raspberry Pi 4 or on
  arm64 KVM with lots of RAM. These should probably all
  migrate to 64-bit kernels with compat user space anyway.
In theory these could also run on a VMSPLIT_4G_4G-like setup,
but I don't think anyone wants to go there. Deprecating highmem
definitely impacts any such users significantly, though staying on
an LTS kernel may be an option if there are only few of them.

The CIP-supported RZ/G1 SoCs can have up to 4 GiB, typically split (even
for 1 GiB or 2 GiB configurations) in two parts, one below and one above
the 32-bit physical limit.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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