Subject: Re: [PATCH v3 0/4] powerpc/mm/radix: Memory unplug fixes

From: Bharata B Rao <[email-masked]>
Date: Tue, 21 Jul 2020 08:59:59 +0530

On Tue, Jul 21, 2020 at 11:45:20AM +1000, Michael Ellerman wrote:
> Nathan Lynch <[email-masked]> writes:
> > "Aneesh Kumar K.V" <[email-masked]> writes:
> >> This is the next version of the fixes for memory unplug on radix.
> >> The issues and the fix are described in the actual patches.
> >
> > I guess this isn't actually causing problems at runtime right now, but I
> > notice calls to resize_hpt_for_hotplug() from arch_add_memory() and
> > arch_remove_memory(), which ought to be mmu-agnostic:
> >
> > int __ref arch_add_memory(int nid, u64 start, u64 size,
> > 			  struct mhp_params *params)
> > {
> > 	unsigned long start_pfn = start >> PAGE_SHIFT;
> > 	unsigned long nr_pages = size >> PAGE_SHIFT;
> > 	int rc;
> >
> > 	resize_hpt_for_hotplug(memblock_phys_mem_size());
> >
> > 	start = (unsigned long)__va(start);
> > 	rc = create_section_mapping(start, start + size, nid,
> > 				    params->pgprot);
> > ...
> 
> Hmm well spotted.
> 
> That does return early if the ops are not setup:
> 
> int resize_hpt_for_hotplug(unsigned long new_mem_size)
> {
> 	unsigned target_hpt_shift;
> 
> 	if (!mmu_hash_ops.resize_hpt)
> 		return 0;
> 
> 
> And:
> 
> void __init hpte_init_pseries(void)
> {
> 	...
> 	if (firmware_has_feature(FW_FEATURE_HPT_RESIZE))
> 		mmu_hash_ops.resize_hpt = pseries_lpar_resize_hpt;
> 
> And that comes in via ibm,hypertas-functions:
> 
> 	{FW_FEATURE_HPT_RESIZE,		"hcall-hpt-resize"},
> 
> 
> But firmware is not necessarily going to add/remove that call based on
> whether we're using hash/radix.

Correct but hpte_init_pseries() will not be called for radix guests.

> 
> So I think a follow-up patch is needed to make this more robust.
> 
> Aneesh/Bharata what platform did you test this series on? I'm curious
> how this didn't break.

I have tested memory hotplug/unplug for radix guest on zz platform and
sanity-tested this for hash guest on P8.

As noted above, mmu_hash_ops.resize_hpt will not be set for radix
guest and hence we won't see any breakage.

However a separate patch to fix this will be good.

Regards,
Bharata.