Re: [PATCH 1/4] arch/*: increase lowmem size to avoid highmem use

["Arnd Bergmann" <arnd@arndb.de>]

On Fri, Dec 19, 2025, at 21:52, Dave Hansen wrote:
> On 12/19/25 12:20, Arnd Bergmann wrote:
>>> But, in the end, I don't this this matters all that much. If you think
>>> having x86 be consistent with ARM, for example, is more important and
>>> ARM really wants this complexity, I can live with it.
>> Yes, I think we do want the default of VMSPLIT_3G_OPT for
>> configs that have neither highmem nor lpae, otherwise the most
>> common embedded configs go from 3072 MiB to 1792 MiB of virtual
>> addressing, and that is much more likely to cause regressions
>> than the 2816 MiB default.
>
> The only thing we'd "regress" would be someone who is repeatedly
> starting from scratch with a defconfig and expecting defconfig to be the
> same all the time. I honestly think that's highly unlikely.

The entire vmsplit selection is guarded by a CONFIG_EXPERT conditional,
so I would expect it to change both for embedded distros that store
a project specific defconfig and for individual users that have a full
.config. If someone sets CONFIG_EXPERT, they do indeed keep any
previous defaults, but I'm also less worried about them.

In the Arm version, the 'choice' statement itself does not depend
on CONFIG_EXPERT, but I've added 'depends on !HIGHMEM || EXPERT'
in VMSPLIT_3G and VMSPLIT_3G_OPT for a similar effect.

> If folks are upgrading and _actually_ exposed to regressions, they've
> got an existing config and won't be hit by these defaults at *all*. They
> won't actually regress.
>
> In other words, I think we can be a lot more aggressive about defaults
> than with the feature set we support. I'd much rather add complexity in
> here for solving a real problem, like if we have armies of 32-bit x86
> users constantly starting new projects from scratch and using defconfigs.
>
> I'd _really_ like to keep the defaults as simple as possible.

I'm fine with 

	default VMSPLIT_2G_OPT if !X86_LPAE
	default VMSPLIT_2G

and dropping the VMSPLIT_3G_OPT default for non-highmem x86
builds if you think that's better. I still think we need the
special case for X86_LPAE/NX users to get to the point of having
VMSPLIT_2G_OPT as the default across architectures for current
HIGHMEM users that have exactly 2GB.

I honestly don't know enough about x86-32 users to have
a good idea who should be optimizing for. The embedded systems
(vortex86 and geode) seem to mostly have 512MB or less and no
PAE, so it probably works either way. As with similar
Arm configurations, these seemed like the highest priority
to me.

I see there are a few rare vortex86dx3 boards and the
upcoming vortex86ex3 that can have 2GB and would be using
HIGHMEM=y but PAE=n today, and these really want the 2G_OPT
split by default, not VMSPLIT_2G.

Hobbyists running on vintage PC systems instead may have
intentionally seeked out the few machines that actually support
2GB or 3.5GB of RAM on late Pentium-M or early Atom CPUs,
though most of the historic machines between the i486 and
the last 32-bit desktops would likely have much less.

    Arnd