Re: [TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

René Herman
2025-09-10T01:06:06Z

> Well, while on x86 1GB systems did use highmem, they'd typically not
> use very much of it.
>
> IOW, they'd have about 900MB as lowmem (ok, I think it was 896MB to
> be exact), with something like 120MB highmem.
>
> So they'd either lose a bit of memory, or they'd use the 2G:2G
> split.
Right, 1G-128M of VMALLOC_RESERVE, but they didn't generally use 2G/2G
but ever since it was introduced late in the 32-bit era VMSPLIT_3G_OPT,
i.e., PAGE_OFFSET = 0xb0000000, i.e., 3G-256M/1G+256M.

Although at the time the patches died an unceremonious death, 4G/4G
patches for x86 were also around; I used to be a (conceptual) fan of
them: it takes quite a number of TLB-flushes to suck more than highmem.

Not one hint of clue if spectre/meltdown is applicable to these systems
and if so, if they in fact do or should already be suffering through
those same flushes anyway but maybe it'd be an option to dig those
patches out of some archive somewhere.

I promise I'll test them on an AMD Duron with 768MB RAM :)

Rene.