linux-kernel - Re: [PATCH v5 02/18] mm: Define __pte_leaf

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Message-ID: <Zmh282yJjxc7zqbL@localhost.localdomain>
Date: Tue, 11 Jun 2024 18:10:27 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Peter Xu <peterx@...hat.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jason Gunthorpe <jgg@...dia.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v5 02/18] mm: Define __pte_leaf_size() to also take a PMD
 entry

On Tue, Jun 11, 2024 at 11:20:01AM -0400, Peter Xu wrote:
> On Tue, Jun 11, 2024 at 05:08:45PM +0200, Oscar Salvador wrote:
> > The problem is that we do not have spare bits for 8xx to mark these ptes
> > as cont-ptes or mark them pte as 8MB, so I do not see a clear path on how
> > we could remove huge_ptep_get for 8xx.
> 
> Right, I remember I thought about this too when I initially looked at one
> previous version of the series, I didn't come up yet with a good solution,
> but I guess we probably need to get rid of hugepd first anyway.  We may
> somehow still need to identify this is a 8M large leaf, and I guess this is
> again the only special case where contpte can go over >1 pmds.

Yes, we definitely need first to get rid of hugepd, which is a huge
step, and one step closer to our goal, but at some point we will have to
see what can we do about 8MB cont-ptes for 8xx and how to mark them,
so ptep_get can work the same way as e.g: arm64
(ptep_get->contpte_ptep_get).

@Christophe: Can you think of a way to flag those ptes? (e.g: a
combination of bits etc)

> I'll leave this to Christophe, but IIUC thp is only PMD_ORDER sized, so
> shouldn't apply to the 8MB pages.

That might be it, yes.


-- 
Oscar Salvador
SUSE Labs