Hi Arnd,
On Thu, Feb 13, 2020 at 5:54 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Wed, Feb 12, 2020 at 9:50 AM Russell King - ARM Linux admin
> <linux@xxxxxxxxxxxxxxx> wrote:
> > On Tue, Feb 11, 2020 at 05:03:02PM -0800, Linus Torvalds wrote:
> > > 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@xxxxxxxxxxxxxx
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
