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

From: Anton Blanchard <[email-masked]>
Date: Fri, 8 Sep 2017 11:15:47 +1000

Hi Reza,

> I may be misunderstanding this, but what if we did something like x86 
> does? When trying to unplug a region smaller than the mapping, they
> fill that part of the pagetable with 0xFD instead of freeing the
> whole thing. Once the whole thing is 0xFD, free it.
> 
> See arch/x86/mm/init_64.c:remove_{pte,pmd,pud}_table()
> 
> ---%<---
> 	memset((void *)addr, PAGE_INUSE, next - addr);
> 
> 	page_addr = page_address(pte_page(*pte));
> 	if (!memchr_inv(page_addr, PAGE_INUSE, PAGE_SIZE)) {
> 		...
> 		pte_clear(&init_mm, addr, pte);
> 		...
> 	}
> ---%<---

But you only have 1GB ptes at this point, you'd need to start
instantiating a new level in the tree, and populate 2MB ptes.

That is what Ben is suggesting. I'm happy to go any way (fix hotplug
to handle this, or increase the memblock size on PowerNV to 1GB), I just
need a solution.

Anton