Re: [PATCH v4 02/15] mm: Make empty_zero_page __ro_after_init

[Mike Rapoport <rppt@kernel.org>]

On Wed, May 13, 2026 at 10:53:08AM +0200, Ard Biesheuvel wrote:
> 
> On Wed, 13 May 2026, at 10:50, Mike Rapoport wrote:
> > On Tue, May 12, 2026 at 02:56:16PM +0200, Ard Biesheuvel wrote:
> >> On Mon, 11 May 2026, at 16:40, Jann Horn wrote:
> >> > On Mon, May 11, 2026 at 10:59 AM Ard Biesheuvel <ardb@kernel.org> wrote:
> >> >> I think we should simply do something along the lines of the below,
> >> >> considering that the size of a data object tends to correlate with
> >> >> its minimum alignment.
> >> >>
> >> >> I do find it rather puzzling that the compiler emits empty_zero_page
> >> >> *after* zero_page_pfn - ideally, we'd combine the below with
> >> >> -fdata-sections so that the linker sees all individual objects, but
> >> >> I suspect that would create some problems elsewhere.
> >> >>
> >> >>
> >> >> --- a/include/asm-generic/vmlinux.lds.h
> >> >> +++ b/include/asm-generic/vmlinux.lds.h
> >> >> @@ -452,7 +452,7 @@
> >> >>  #define RO_AFTER_INIT_DATA                                   \
> >> >>         . = ALIGN(8);                                         \
> >> >>         __start_ro_after_init = .;                            \
> >> >> -       *(.data..ro_after_init)                               \
> >> >> +       *(SORT_BY_ALIGNMENT(.data..ro_after_init))            \
> >> >
> >> > Oh, neat, I didn't realize that's possible. That seems like a nicer
> >> > approach...
> >> 
> >> Neat but rather ineffective, unfortunately. (I don't see a size
> >> difference with the arm64 defconfig kernel)
> >> 
> >> Given that empty_zero_page only ever gets its address taken, we
> >> might just move it into the linker script if that requires tweaking
> >> anyway. We can just place it at the start of .rodata, which is
> >> already page aligned on most architectures (and will become page
> >> aligned unless EMPTY_ZERO_PAGE is #define'd by the arch linker
> >> script to something else)
> >> 
> >> 
> >> --- a/include/asm-generic/vmlinux.lds.h
> >> +++ b/include/asm-generic/vmlinux.lds.h
> >> @@ -472,6 +472,17 @@
> >>  #endif
> >>  #endif
> >>  
> >> +#ifndef EMPTY_ZERO_PAGE
> >> +#ifndef __HAVE_COLOR_ZERO_PAGE
> >
> > I don't think we want let architectures that don't use colored zero pages
> > redefine it.
> > If it will be really required we can add the ability to redefine
> > EMPTY_ZERO_PAGE later.
> >
> 
> I was actually intending to add use this for arm64 in the next patch. It
> already has a reserved_pg_dir in .rodata which is page-sized (i.e., up
> to 64k in size) and guaranteed to remain all zeroes, so empty_zero_page
> could actually be an alias for that. This is what I had in a previous
> revision, before you turned the empty_zero_page definition into common
> code:

Works for me if from arm64 perspective that's ok :)
 
> https://lore.kernel.org/all/20260320145934.2349881-16-ardb+git@google.com/

-- 
Sincerely yours,
Mike.