Re: [PATCH] m68k: fix for systems with memory at end of 32-bit address space

[Michael Schmitz <schmitzmic@gmail.com>]

Hi Geert,

Am 25.02.2023 um 00:01 schrieb Geert Uytterhoeven:
>> Now I just need to figure out why the last page of memory is excluded.
>> Do you also see this on other machines?
>>
>> Zone ranges:
>>   DMA      [mem 0x00000000fc000000-0x00000fffffffefff]
>>   Normal   empty
>> Movable zone start for each node
>> Early memory node ranges
>>   node   0: [mem 0x00000000fc000000-0x00000000ffffefff]
>> Initmem setup node 0 [mem 0x00000000fc000000-0x00000000ffffefff]
>
> I don't see that on ARAnyM or on qemu-system-m68k, but those
> don't have memory at the end of the address space.
>
> ISTR something about not mapping the last page of RAM, if it's at the
> end of the address space, to avoid wrap-around while prefetching?

What would that achieve? The prefetch would still hit an unmapped page.

I can see that excluding the last page from use by memory allocators 
will prevent wrap, but that last page must then still be mapped, no?

Cheers,

	Michael

>
> [searching]
>
> I think it's a side-effect of:
>
>     static inline phys_addr_t memblock_cap_size(phys_addr_t base,
> phys_addr_t *size)
>     {
>             return *size = min(*size, PHYS_ADDR_MAX - base);
>     }
>
> with
>
>     #define PHYS_ADDR_MAX  (~(phys_addr_t)0)
>
> 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
>