linux-kernel - Re: [PATCH 3.5] arm: make initialization of zero page independent of the memory map (was Re: [PATCH v2 22/28] arch, mm: consolidate initialization of nodes, zones and memory map)

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Message-ID: <aVrUDeSkqqY9ZCtS@shell.armlinux.org.uk>
Date: Sun, 4 Jan 2026 20:56:45 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Mike Rapoport <rppt@...nel.org>
Cc: Klara Modin <klarasmodin@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH 3.5] arm: make initialization of zero page independent of
 the memory map (was Re: [PATCH v2 22/28] arch, mm: consolidate
 initialization of nodes, zones and memory map)

On Sun, Jan 04, 2026 at 02:01:40PM +0200, Mike Rapoport wrote:
> From 35d016bbf5da7c08cc5c5547c85558fc50cb63aa Mon Sep 17 00:00:00 2001
> From: Klara Modin <klarasmodin@...il.com>
> Date: Sat, 3 Jan 2026 20:40:09 +0200
> Subject: [PATCH] arm: make initialization of zero page independent of the
>  memory map
> 
> Unlike most architectures, arm keeps a struct page pointer to the
> empty_zero_page and to initialize it requires conversion of a virtual
> address to page which makes it necessary to have memory map initialized
> before creating the empty_zero_page.
> 
> Make empty_zero_page a stataic array in BSS to decouple it's
> initialization from the initialization of the memory map.

I see you haven't considered _why_ ARM does this.

You are getting rid of the flush_dcache_page() call, which ensures
that the zeroed contents of the page are pushed out of the cache
into memory. This is necessary.

BSS is very similar. It's memset() during the kernel boot _after_
the caches are enabled. Without an explicit flush, nothing
guarantees that those writes will be visible to userspace.

To me, this seems like a bad idea, which will cause userspace to
break.

We need to call flush_dcache_page(), and _that_ requires a struct
page.

-- 
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