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

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Message-Id: <55cbc329-4b14-4f62-90bb-1e1007cc1152@app.fastmail.com>
Date: Wed, 10 Sep 2025 17:13:57 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Matthew Wilcox" <willy@...radead.org>,
 "Linus Walleij" <linus.walleij@...aro.org>
Cc: 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>,
 "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>,
 "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 Wed, Sep 10, 2025, at 16:04, Matthew Wilcox wrote:
> On Wed, Sep 10, 2025 at 03:10:05PM +0200, Linus Walleij wrote:
>> 
>> This in practice means the area 0xf1000000-0xffffffff
>> which also includes the high exception vectors that must for
>> natural reasons also be kept in the mapping.
>> What we have mapped today (all of the time) is
>> 0xc0000000-0xffffffff.
>> 
>> A minimal 256 MB mapping from 0xf0000000-0xffffffff
>> should be able to cover this.
>
> Can I ask where the MMIO (PCI?) window lives?

That 256MB area is shared by anything the kernel can access
directly, other than the linear map: vmlinux, modules, vmalloc,
ioremap, vmap-stack, etc.

This is mostly unchanged from the tradtitional layout on
arm32 that has the same ~256MB limitation, though it does
get a little tighter because .text/.data is now also
mapped into this space instead of executed from the linear
map.

      ARnd