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

["Ard Biesheuvel" <ardb@kernel.org>]

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
+#define EMPTY_ZERO_PAGE                                        \
+       . = ALIGN(PAGE_SIZE);                                   \
+       empty_zero_page = .;                                    \
+       . += PAGE_SIZE;
+#else
+#define EMPTY_ZERO_PAGE
+#endif
+#endif
+
 /*
  * Read only Data
  */
@@ -479,6 +490,7 @@
        . = ALIGN((align));                                     \
        .rodata           : AT(ADDR(.rodata) - LOAD_OFFSET) {   \
                __start_rodata = .;                             \
+               EMPTY_ZERO_PAGE                                 \
                *(.rodata) *(.rodata.*) *(.data.rel.ro*)        \
                SCHED_DATA                                      \