Re: [PATCH] arm64: mm: Correct the update of max_pfn

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Mar 21, 2025 at 03:00:19PM +0800, Zhenhua Huang wrote:
> Hotplugged memory can be smaller than the original memory. For example,
> on my target:
> 
> root@genericarmv8:~# cat /sys/kernel/debug/memblock/memory
>    0: 0x0000000064005000..0x0000000064023fff    0 NOMAP
>    1: 0x0000000064400000..0x00000000647fffff    0 NOMAP
>    2: 0x0000000068000000..0x000000006fffffff    0 DRV_MNG
>    3: 0x0000000088800000..0x0000000094ffefff    0 NONE
>    4: 0x0000000094fff000..0x0000000094ffffff    0 NOMAP
> max_pfn will affect read_page_owner. Therefore, it should first compare and
> then select the larger value for max_pfn.
> 
> Fixes: 8fac67ca236b ("arm64: mm: update max_pfn after memory hotplug")
> Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
> ---
>  arch/arm64/mm/mmu.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 1dfe1a8efdbe..310ff75891ef 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1361,7 +1361,8 @@ int arch_add_memory(int nid, u64 start, u64 size,
>  		__remove_pgd_mapping(swapper_pg_dir,
>  				     __phys_to_virt(start), size);
>  	else {
> -		max_pfn = PFN_UP(start + size);
> +		/* Address of hotplugged memory can be smaller */
> +		max_pfn = max(max_pfn, PFN_UP(start + size));
>  		max_low_pfn = max_pfn;
>  	}

I think this makes sense and also matches what x86 does. I'll queue it
at -rc1 as a fix.

Thanks.

-- 
Catalin