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

[Nicholas Piggin <npiggin@gmail.com>]

On Mon, 30 Apr 2018 18:34:32 +1000
Balbir Singh <bsingharora@gmail.com> wrote:

> This reverts commit 53ecde0b9126ff140abe3aefd7f0ec64d6fa36b0.
> 
> The commit above changed the memblock size to 1GiB, which did some
> nice things like create fewer TLB entries for mapping memory at
> the time of hotplug. The downside is that it changes the granularity
> at which memory can be hot-plugged and hot-unplugged. The implication
> is that if we had less than a 1GiB to hot-plug/hot-unplug that
> would not be possible.
> 
> The reason we had this fix was to resolve an issue where we did not
> split mappings on hot-unplug, leaving a TLB entry that spanned the
> region that was unplugged. This is now fixed by 4dd5f8a99e79 which
> splits the page table, removing the MMU mappings for the hot-unplugged
> region correctly.
> 
> This trade-offs performance as we increase the number of TLB's to
> map 1GiB of memory, mapped in via hot-plug, but gives us the ability
> to handle smaller memory sizes.

Why does powernv have any memory block size override at all now?

> 
> Signed-off-by: Balbir Singh <bsingharora@gmail.com>
> Signed-off-by: Michael Neuling <mikey@neuling.org>
> ---
>  arch/powerpc/platforms/powernv/setup.c | 10 +---------
>  1 file changed, 1 insertion(+), 9 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/powernv/setup.c b/arch/powerpc/platforms/powernv/setup.c
> index ef8c9ce53a61..fa63d3fff14c 100644
> --- a/arch/powerpc/platforms/powernv/setup.c
> +++ b/arch/powerpc/platforms/powernv/setup.c
> @@ -356,15 +356,7 @@ static void pnv_kexec_cpu_down(int crash_shutdown, int secondary)
>  #ifdef CONFIG_MEMORY_HOTPLUG_SPARSE
>  static unsigned long pnv_memory_block_size(void)
>  {
> -	/*
> -	 * 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;
> +	return 256UL * 1024 * 1024;
>  }
>  #endif
>