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

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

On Fri, Dec 19, 2025, at 19:02, Dave Hansen wrote:
> On 12/19/25 08:15, Arnd Bergmann wrote:
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -1416,7 +1416,9 @@ config HIGHMEM4G
>>  
>>  choice
>>  	prompt "Memory split" if EXPERT
>> -	default VMSPLIT_3G
>> +	default VMSPLIT_2G_OPT if HIGHMEM && !X86_PAE
>> +	default VMSPLIT_2G if X86_PAE
>> +	default VMSPLIT_3G_OPT
>>  	depends on X86_32
>
> For simplicity, I think this can just be:
>
> -	default VMSPLIT_3G
> +	default VMSPLIT_2G
>
> I doubt the 2G vs. 2G_OPT matters in very many cases. If it does, folks
> can just set it in their config manually.
>
> 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.

It would be nice to not need the VMSPLIT_2G default for PAE/LPAE,
but that seems like a larger change.

     Arnd