Hi Catalin, James,
sorry for the late reply but I got sidetracked.

On Fri, 2020-11-13 at 11:29 +0000, Catalin Marinas wrote:
[...]
> > > > Let me stress that knowing the DMA constraints in the system before reserving
> > > > crashkernel's regions is necessary if we ever want it to work seamlessly on all
> > > > platforms. Be it small stuff like the Raspberry Pi or huge servers with TB of
> > > > memory.
> > > 
> > > Indeed. So we have 3 options (so far):
> > > 
> > > 1. Allow the crashkernel reservation to go into the linear map but set
> > >    it to invalid once allocated.
> > > 
> > > 2. Parse the flattened DT (not sure what we do with ACPI) before
> > >    creating the linear map. We may have to rely on some SoC ID here
> > >    instead of actual DMA ranges.
> > > 
> > > 3. Assume the smallest ZONE_DMA possible on arm64 (1GB) for crashkernel
> > >    reservations and not rely on arm64_dma_phys_limit in
> > >    reserve_crashkernel().
> > > 
> > > I think (2) we tried hard to avoid. Option (3) brings us back to the
> > > issues we had on large crashkernel reservations regressing on some
> > > platforms (though it's been a while since, they mostly went quiet ;)).
> > > However, with Chen's crashkernel patches we end up with two
> > > reservations, one in the low DMA zone and one higher, potentially above
> > > 4GB. Having a fixed 1GB limit wouldn't be any worse for crashkernel
> > > reservations than what we have now.
> > > 
> > > If (1) works, I'd go for it (James knows this part better than me),
> > > otherwise we can go for (3).
> > 
> > Overall, I'd prefer (1) as well, and I'd be happy to have a got at it. If not
> > I'll append (3) in this series.
> 
> I think for 1 we could also remove the additional KEXEC_CORE checks,
> something like below, untested:
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 3e5a6913acc8..27ab609c1c0c 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -477,7 +477,8 @@ static void __init map_mem(pgd_t *pgdp)
>  	int flags = 0;
>  	u64 i;
>  
> -	if (rodata_full || debug_pagealloc_enabled())
> +	if (rodata_full || debug_pagealloc_enabled() ||
> +	    IS_ENABLED(CONFIG_KEXEC_CORE))
>  		flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>  
>  	/*
> @@ -487,11 +488,6 @@ static void __init map_mem(pgd_t *pgdp)
>  	 * the following for-loop
>  	 */
>  	memblock_mark_nomap(kernel_start, kernel_end - kernel_start);
> -#ifdef CONFIG_KEXEC_CORE
> -	if (crashk_res.end)
> -		memblock_mark_nomap(crashk_res.start,
> -				    resource_size(&crashk_res));
> -#endif
>  
>  	/* map all the memory banks */
>  	for_each_mem_range(i, &start, &end) {
> @@ -518,21 +514,6 @@ static void __init map_mem(pgd_t *pgdp)
>  	__map_memblock(pgdp, kernel_start, kernel_end,
>  		       PAGE_KERNEL, NO_CONT_MAPPINGS);
>  	memblock_clear_nomap(kernel_start, kernel_end - kernel_start);
> -
> -#ifdef CONFIG_KEXEC_CORE
> -	/*
> -	 * Use page-level mappings here so that we can shrink the region
> -	 * in page granularity and put back unused memory to buddy system
> -	 * through /sys/kernel/kexec_crash_size interface.
> -	 */
> -	if (crashk_res.end) {
> -		__map_memblock(pgdp, crashk_res.start, crashk_res.end + 1,
> -			       PAGE_KERNEL,
> -			       NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS);
> -		memblock_clear_nomap(crashk_res.start,
> -				     resource_size(&crashk_res));
> -	}
> -#endif
>  }
>  
>  void mark_rodata_ro(void)

So as far as I'm concerned this is good enough for me. I took the time to
properly test crashkernel on RPi4 using the series, this patch, and another
small fix to properly update /proc/iomem.

I'll send v7 soon, but before, James (or anyone for that matter) any obvious
push-back to Catalin's solution?

Regards,
Nicolas