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Date: Wed, 20 Mar 2024 16:24:25 -0400
From: Peter Xu <peterx@redhat.com>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	Jason Gunthorpe <jgg@nvidia.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>,
	Matthew Wilcox <willy@infradead.org>,
	"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"x86@kernel.org" <x86@kernel.org>, Mike Rapoport <rppt@kernel.org>,
	Muchun Song <muchun.song@linux.dev>,
	"sparclinux@vger.kernel.org" <sparclinux@vger.kernel.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>,
	"Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Subject: Re: [PATCH 09/13] mm/powerpc: Redefine pXd_huge() with pXd_leaf()
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On Wed, Mar 20, 2024 at 05:40:39PM +0000, Christophe Leroy wrote:
> 
> 
> Le 20/03/2024 à 17:09, Peter Xu a écrit :
> > On Wed, Mar 20, 2024 at 06:16:43AM +0000, Christophe Leroy wrote:
> >> At the first place that was to get a close fit between hardware
> >> pagetable topology and linux pagetable topology. But obviously we
> >> already stepped back for 512k pages, so let's go one more step aside and
> >> do similar with 8M pages.
> >>
> >> I'll give it a try and see how it goes.
> > 
> > So you're talking about 8M only for 8xx, am I right?
> 
> Yes I am.
> 
> > 
> > There seem to be other PowerPC systems use hugepd.  Is it possible that we
> > convert all hugepd into cont_pte form?
> 
> Indeed.
> 
> Seems like we have hugepd for book3s/64 and for nohash.
> 
> For book3s I don't know, may Aneesh can answer.
> 
> For nohash I think it should be possible because TLB misses are handled 
> by software. Even the e6500 which has a hardware tablewalk falls back on 
> software walk when it is a hugepage IIUC.

It'll be great if I can get some answer here, and then I know the path for
hugepd in general.  I don't want to add any new code into core mm to
something destined to fade away soon.

One option for me is I can check a macro of hugepd existance, so all new
code will only work when hugepd is not supported on such arch.  However
that'll start to make some PowerPC systems special (which I still tried
hard to avoid, if that wasn't proved in the past..), meanwhile we'll also
need to keep some generic-mm paths (that I can already remove along with
the new code) only for these hugepd systems.  But it's still okay to me,
it'll be just a matter of when to drop those codes, sooner or later.

Thanks,

-- 
Peter Xu