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Subject: Re: [PATCH] powerpc/mm: update arch_{add,remove}_memory() for radix
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
 Reza Arbab <arbab@linux.vnet.ibm.com>,
 Benjamin Herrenschmidt <benh@kernel.crashing.org>,
 Paul Mackerras <paulus@samba.org>, Michael Ellerman <mpe@ellerman.id.au>,
 Dan Williams <dan.j.williams@intel.com>,
 Gavin Shan <gwshan@linux.vnet.ibm.com>,
 David Gibson <david@gibson.dropbear.id.au>,
 Vasant Hegde <hegdevasant@linux.vnet.ibm.com>,
 Scott Wood <scottwood@freescale.com>, Oliver O'Halloran <oohall@gmail.com>,
 linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
References: <1466699962-22412-1-git-send-email-arbab@linux.vnet.ibm.com>
 <87mvmbygdb.fsf@skywalker.in.ibm.com>
From: Balbir Singh <bsingharora@gmail.com>
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Date: Fri, 24 Jun 2016 13:22:37 +1000
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On 24/06/16 03:17, Aneesh Kumar K.V wrote:
> Reza Arbab <arbab@linux.vnet.ibm.com> writes:
> 
>> These functions are making direct calls to the hash table APIs,
>> leading to a BUG() on systems using radix.
>>
>> Switch them to the vmemmap_{create,remove}_mapping() wrappers, and
>> move to the __meminit section.
> 
> 
> They are really not the same. They can possibly end up using different
> base page size. Also vmemmap is available only with SPARSEMEM_VMEMMAP
> enabled. Does hotplug depend on sparsemem vmemmap ?

# eventually, we can have this option just 'select SPARSEMEM'
config MEMORY_HOTPLUG
        bool "Allow for memory hot-add"
        depends on SPARSEMEM || X86_64_ACPI_NUMA
        depends on ARCH_ENABLE_MEMORY_HOTPLUG

We depend on sparsemem for sure. vmemmap is just a way of getting the memory
virtually mapped. From the patch perspective, I think we need the equivalent of
just mapping the pages in kernel. The address may differ based on whether vmemmap
is used or not and of-course page_size, 

Balbir Singh