linux-kernel - Re: [TECH TOPIC] Reaching consensus on CONFIG

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Message-Id: <598ab56d-7dd0-4adb-b0fe-30ad5cf24335@app.fastmail.com>
Date: Mon, 22 Sep 2025 23:22:30 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Nicolas Schichan" <nschichan@...ebox.fr>
Cc: "Jason Gunthorpe" <jgg@...dia.com>,
 "Geert Uytterhoeven" <geert@...ux-m68k.org>, ksummit@...ts.linux.dev,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, linux-mips@...r.kernel.org,
 linux-mm@...ck.org, imx@...ts.linux.dev,
 "Christophe Leroy" <christophe.leroy@...roup.eu>,
 "Richard Weinberger" <richard@....at>,
 "Lucas Stach" <l.stach@...gutronix.de>,
 "Linus Walleij" <linus.walleij@...aro.org>,
 "Geert Uytterhoeven" <geert+renesas@...der.be>,
 "Ankur Arora" <ankur.a.arora@...cle.com>,
 "David Hildenbrand" <david@...hat.com>,
 "Mike Rapoport" <rppt@...nel.org>,
 "Lorenzo Stoakes" <lorenzo.stoakes@...cle.com>,
 "Matthew Wilcox" <willy@...radead.org>,
 "Andrew Morton" <akpm@...ux-foundation.org>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 "Vlastimil Babka" <vbabka@...e.cz>,
 "Suren Baghdasaryan" <surenb@...gle.com>,
 "Ira Weiny" <ira.weiny@...el.com>, "Nishanth Menon" <nm@...com>,
 Heiko Stübner <heiko@...ech.de>,
 "Alexander Sverdlin" <alexander.sverdlin@...il.com>,
 "Chester A. Unal" <chester.a.unal@...nc9.com>,
 "Sergio Paracuellos" <sergio.paracuellos@...il.com>,
 "Andreas Larsson" <andreas@...sler.com>
Subject: Re: [TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

On Mon, Sep 22, 2025, at 19:05, Nicolas Schichan wrote:
>> On Fri, Sep 19, 2025, at 16:34, Jason Gunthorpe wrote:
>> > On Fri, Sep 19, 2025 at 04:22:20PM +0200, Arnd Bergmann wrote:
>>
>> I did get an email from Nicolas Schichan (added to Cc here),
>> and he is still supporting a widely deployed Kirkwood based
>> platform that uses 1GB RAM configurations. He should get
>> a chance to test that with CONFIG_VMSPLIT_3G_OPT, but I
>> would expect that to continue working, possibly with minor
>> bugfixes.
>
> We don't use HIGMEM on our Kirkwood platform, we are happy using a
> 2G/2G WMSPLIT. We don't need a lot of virtual address space for
> userland, and with the 2G split we don't waste physical memory.

Ok, good.

> I'm happy to test your patch serie with VMSPLIT_3G_OPT and see if it
> still boots with it once you send it.

The patches I did for turning off HIGHMEM on ARMv5 won't have
any effect if you are already running without highmem, so I
don't think we need to wait for your tests specifically.
Any testing would be useful of course just to check that
everything still works.

Jason, I think we can go ahead with your suggestion. I can
post my patches for review once I've done some testing on
them.

     Arnd