linux-kernel - Re: [RFC PATCH 0/8] Reimplement huge pages without hugepd on powerpc 8xx

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Message-ID: <ZhgMqF7SNaISrYMJ@x1n>
Date: Thu, 11 Apr 2024 12:15:36 -0400
From: Peter Xu <peterx@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH 0/8] Reimplement huge pages without hugepd on powerpc
 8xx

On Mon, Mar 25, 2024 at 01:38:40PM -0300, Jason Gunthorpe wrote:
> On Mon, Mar 25, 2024 at 03:55:53PM +0100, Christophe Leroy wrote:
> > This series reimplements hugepages with hugepd on powerpc 8xx.
> > 
> > Unlike most architectures, powerpc 8xx HW requires a two-level
> > pagetable topology for all page sizes. So a leaf PMD-contig approach
> > is not feasible as such.
> > 
> > Possible sizes are 4k, 16k, 512k and 8M.
> > 
> > First level (PGD/PMD) covers 4M per entry. For 8M pages, two PMD entries
> > must point to a single entry level-2 page table. Until now that was
> > done using hugepd. This series changes it to use standard page tables
> > where the entry is replicated 1024 times on each of the two pagetables
> > refered by the two associated PMD entries for that 8M page.
> > 
> > At the moment it has to look into each helper to know if the
> > hugepage ptep is a PTE or a PMD in order to know it is a 8M page or
> > a lower size. I hope this can me handled by core-mm in the future.
> > 
> > There are probably several ways to implement stuff, so feedback is
> > very welcome.
> 
> I thought it looks pretty good!

I second it.

I saw the discussions in patch 1.  Christophe, I suppose you're exploring
the big hammer over hugepd, and perhaps went already with the 32bit pmd
solution for nohash/32bit challenge you mentioned?

I'm trying to position my next step; it seems like at least I should not
adding any more hugepd code, then should I go with ARCH_HAS_HUGEPD checks,
or you're going to have an RFC soon then I can base on top?

Thanks,

-- 
Peter Xu