[PATCH] powerpc/powernv: Increase memory block size to 1GB on radix

[Aneesh Kumar K.V]

On 09/07/2017 10:35 AM, Anton Blanchard wrote:
> From: Anton Blanchard <anton at samba.org>
> 
> Memory hot unplug on PowerNV radix hosts is broken. Our memory block
> size is 256MB but since we map the linear region with very large pages,
> each pte we tear down maps 1GB.
> 
> A hot unplug of one 256MB memory block results in 768MB of memory
> getting unintentionally unmapped. At this point we are likely to oops.
> 
> Fix this by increasing our memory block size to 1GB on PowerNV radix
> hosts.
> 
> Signed-off-by: Anton Blanchard <anton at samba.org>
> ---
>   arch/powerpc/platforms/powernv/setup.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/platforms/powernv/setup.c b/arch/powerpc/platforms/powernv/setup.c
> index 897aa1400eb8..bbb73aa0eb8f 100644
> --- a/arch/powerpc/platforms/powernv/setup.c
> +++ b/arch/powerpc/platforms/powernv/setup.c
> @@ -272,7 +272,15 @@ static void pnv_kexec_cpu_down(int crash_shutdown, int secondary)
>   #ifdef CONFIG_MEMORY_HOTPLUG_SPARSE
>   static unsigned long pnv_memory_block_size(void)
>   {
> -	return 256UL * 1024 * 1024;
> +	/*
> +	 * We map the kernel linear region with 1GB large pages on radix. For
> +	 * memory hot unplug to work our memory block size must be at least
> +	 * this size.
> +	 */
> +	if (radix_enabled())
> +		return 1UL * 1024 * 1024 * 1024;
> +	else
> +		return 256UL * 1024 * 1024;
>   }
>   #endif
> 

There is a similar issue being worked on w.r.t pseries.

https://lkml.kernel.org/r/1502357028-27465-1-git-send-email-bharata@linux.vnet.ibm.com

The question is should we map these regions ? ie, we need to tell the 
kernel memory region that we would like to hot unplug later so that we 
avoid doing kernel allocations from that. If we do that, then we can 
possibly map them via 2M size ?

-aneesh