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

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Date: Fri, 12 Apr 2024 14:08:03 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Peter Xu <peterx@...hat.com>, Jason Gunthorpe <jgg@...dia.com>
CC: Andrew Morton <akpm@...ux-foundation.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [RFC PATCH 0/8] Reimplement huge pages without hugepd on powerpc
 8xx



Le 11/04/2024 à 18:15, Peter Xu a écrit :
> 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?

Depends on what you expect by "soon".

I sure won't be able to send any RFC before end of April.

Should be possible to have something during May.

Christophe