linux-kernel - Re: [PATCH] powerpc/mm: update arch_{add,remove}

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Message-ID: <1467112865.16036.4.camel@ellerman.id.au>
Date:	Tue, 28 Jun 2016 21:21:05 +1000
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Reza Arbab <arbab@...ux.vnet.ibm.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Balbir Singh <bsingharora@...il.com>,
	Gavin Shan <gwshan@...ux.vnet.ibm.com>,
	David Gibson <david@...son.dropbear.id.au>,
	Vasant Hegde <hegdevasant@...ux.vnet.ibm.com>,
	Scott Wood <scottwood@...escale.com>,
	Oliver O'Halloran <oohall@...il.com>,
	Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/mm: update arch_{add,remove}_memory() for radix

On Thu, 2016-06-23 at 14:37 -0500, Reza Arbab wrote:
> On Thu, Jun 23, 2016 at 10:47:20PM +0530, Aneesh Kumar K.V wrote:
> > Reza Arbab <arbab@...ux.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 ?
> 
> I'm not sure. Maybe it's best if I back up a step and explain what lead 
> me to this patch. During hotplug, you get
> 
> ...
> 	arch_add_memory
> 		create_section_mapping
> 			htab_bolt_mapping
> 				BUG_ON(!ppc_md.hpte_insert);
> 
> So it seemed to me that I needed a radix equivalent of 
> create_section_mapping().
> 
> After some digging, I found hash__vmemmap_create_mapping() and 
> radix__vmemmap_create_mapping() did what I needed. I did not notice the 
> #ifdef SPARSEMEM_VMEMMAP around them.

I think that's more by luck than design. The vmemmap routines use
mmu_vmemmap_psize which is probably but not definitely the same as
mmu_linear_psize.

> Could it be that the functions just need to be renamed 
> hash__create_mapping()/radix__create_mapping() and moved out of #ifdef 
> SPARSEMEM_VMEMMAP?

No, you need to use mmu_linear_psize for the hotplug case.

But you can probably factor out a common routine that both cases use, and hide
the hash vs radix check in that.

And probably send me a patch to make MEMORY_HOTPLUG depend on !RADIX for v4.7?

cheers