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

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Message-ID: <20240325163840.GF6245@nvidia.com>
Date: Mon, 25 Mar 2024 13:38:40 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Peter Xu <peterx@...hat.com>,
	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 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!

Thanks,
Jason