Re: [PATCH v22 4/9] arm64: kdump: Don't force page-level mappings for memory above 4G

Catalin Marinas
2022-04-26T14:26:41Z

On Thu, Apr 14, 2022 at 07:57:15PM +0800, Zhen Lei wrote:
> @@ -540,13 +540,31 @@ static void __init map_mem(pgd_t *pgdp)
> for_each_mem_range(i, &start, &end) {
> if (start >= end)
> break;
> +
> +#ifdef CONFIG_KEXEC_CORE
> + if (eflags && (end >= SZ_4G)) {
> + /*
> + * The memory block cross the 4G boundary.
> + * Forcibly use page-level mappings for memory under 4G.
> + */
> + if (start < SZ_4G) {
> + __map_memblock(pgdp, start, SZ_4G - 1,
> + pgprot_tagged(PAGE_KERNEL), flags | eflags);
> + start = SZ_4G;
> + }
> +
> + /* Page-level mappings is not mandatory for memory above 4G */
> + eflags = 0;
> + }
> +#endif

That's a bit tricky if a SoC has all RAM above 4G. IIRC AMD Seattle had
this layout. See max_zone_phys() for how we deal with this, basically
extending ZONE_DMA to the whole range if RAM starts above 4GB. In that
case, crashkernel reservation would fall in the range above 4GB.

BTW, we changed the max_zone_phys() logic with commit 791ab8b2e3db
("arm64: Ignore any DMA offsets in the max_zone_phys() calculation").

--
Catalin