On Sun, Jan 04, 2026 at 08:56:45PM +0000, Russell King (Oracle) wrote:

On Sun, Jan 04, 2026 at 02:01:40PM +0200, Mike Rapoport wrote:

quoted

From 35d016bbf5da7c08cc5c5547c85558fc50cb63aa Mon Sep 17 00:00:00 2001
From: Klara Modin <redacted>
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.

There's a call to flush_cache_all() paging_init()->devicemaps_init() that
will guarantee that those writes are flushed long before userspace starts.
 

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.

Right now there's __flush_dcache_folio() that will break anyway when folio
divorces from struct page.   

-- 
Sincerely yours,
Mike.