On Tue, 27 Jan 2026 at 10:34, Ryan Roberts <[email protected]> wrote:
>
> On 26/01/2026 09:26, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <[email protected]>
> >
> > The zero page should contain only zero bytes, and so mapping it
> > read-write is unnecessary. Combine it with reserved_pg_dir, which lives
> > in the read-only region of the kernel, and already serves a similar
> > purpose.
> >
> > Signed-off-by: Ard Biesheuvel <[email protected]>
> > ---
> > arch/arm64/kernel/vmlinux.lds.S | 1 +
> > arch/arm64/mm/mmu.c | 3 +--
> > 2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/vmlinux.lds.S
> > b/arch/arm64/kernel/vmlinux.lds.S
> > index ad6133b89e7a..b2a093f5b3fc 100644
> > --- a/arch/arm64/kernel/vmlinux.lds.S
> > +++ b/arch/arm64/kernel/vmlinux.lds.S
> > @@ -229,6 +229,7 @@ SECTIONS
> > #endif
> >
> > reserved_pg_dir = .;
> > + empty_zero_page = .;
> > . += PAGE_SIZE;
> >
> > swapper_pg_dir = .;
>
> Isn't there a magic macro for getting from swapper to reserved? That will need
> updating?
>
Why? This just adds an alias to refer to the same allocation.
> /*
> * Open-coded (swapper_pg_dir - reserved_pg_dir) as this cannot be calculated
> * until link time.
> */
> #define RESERVED_SWAPPER_OFFSET (PAGE_SIZE)
>
>
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index 9ae7ce00a7ef..c36422a3fae2 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -66,9 +66,8 @@ long __section(".mmuoff.data.write")
> > __early_cpu_boot_status;
> >
> > /*
> > * Empty_zero_page is a special page that is used for zero-initialized data
> > - * and COW.
> > + * and COW. Defined in the linker script.
> > */
> > -unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]
> > __page_aligned_bss;
> > EXPORT_SYMBOL(empty_zero_page);
>
> What's the benefit of giving it it's own place in the linker script vs just
> declaring it as const and having it placed in the rodata?
>
Because it collapses the two into one.
