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Subject: Re: [PATCH v3 0/4] powerpc/mm/radix: Memory unplug fixes
To: Michael Ellerman <mpe@ellerman.id.au>, Nathan Lynch <nathanl@linux.ibm.com>
References: <20200709131925.922266-1-aneesh.kumar@linux.ibm.com>
 <87r1tb1rw2.fsf@linux.ibm.com> <87tuy1sksv.fsf@mpe.ellerman.id.au>
From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Message-ID: <0f79bf71-132e-6f3b-9b51-fb893b0dd451@linux.ibm.com>
Date: Tue, 21 Jul 2020 10:12:42 +0530
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Cc: linuxppc-dev@lists.ozlabs.org, Bharata B Rao <bharata@linux.ibm.com>
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On 7/21/20 7:15 AM, Michael Ellerman wrote:
> Nathan Lynch <nathanl@linux.ibm.com> writes:
>> "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> 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.
> 


We are good there because hpte_init_pseries is only called for hash 
translation.

early_init_mmu()
-> hash__early_init_mmu
    -> hpte_init_pseries
       -> mmu_hash_ops.resize_hpt = pseries_lpar_resize_hpt;

> 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.

All the changes are tested with kvm.

-aneesh