Re: [RFC/RFT PATCH 03/19] x86: Combine .data with .bss in kernel mapping

Borislav Petkov
2026-03-06T19:07:54Z

On Thu, Jan 08, 2026 at 09:25:30AM +0000, Ard Biesheuvel wrote:
> The primary mapping of the kernel image is made using huge pages where
> possible, mostly to minimize TLB pressure (Only the entry text section
> requires alignment to 2 MiB). This involves some rounding and padding of
> the .text and .rodata sections, resulting in gaps. These gaps are
> smaller than a huge page, and are remapped using different permissions,
> resulting in fragmentation of the huge page mappings at the edges of
> those regions.
>
> Similarly, there is a gap between .data and .bss, where the init text
> and data regions reside. This means that the end of the .data region and
> the start of the .bss region are not covered by huge page mappings
> either, even though both regions use the same permissions (RW+NX).
>
> Improve the situation, by placing .data and .bss adjacently in the
> linker map, and putting the init text and data regions after .rodata,
> taking the place of the rodata/data gap. This results in one fewer gap,
> and a more efficient mapping of the .data and .bss regions.
>
> To preserve the x86_64 ELF layout with PT_LOAD regions aligned to 2 MiB,
> start the second ELF segment at .init.data and align it to 2 MiB. The
> resulting padding will be covered by the init region and will be freed
> along with it after boot.
>
> defconfig + Clang 19:
>
> Before:
>
> 0xffffffff81000000-0xffffffff82200000 18M ro PSE GLB x pmd
> 0xffffffff82200000-0xffffffff8231c000 1136K ro GLB x pte
> 0xffffffff8231c000-0xffffffff82400000 912K RW GLB NX pte
> 0xffffffff82400000-0xffffffff82a00000 6M ro PSE GLB NX pmd
> 0xffffffff82a00000-0xffffffff82b40000 1280K ro GLB NX pte
> 0xffffffff82b40000-0xffffffff82c00000 768K RW GLB NX pte
> 0xffffffff82c00000-0xffffffff83400000 8M RW PSE GLB NX pmd
> 0xffffffff83400000-0xffffffff83800000 4M RW GLB NX pte
>
> After:
>
> 0xffffffff81000000-0xffffffff82200000 18M ro PSE GLB x pmd
> 0xffffffff82200000-0xffffffff8231c000 1136K ro GLB x pte
> 0xffffffff8231c000-0xffffffff82400000 912K RW GLB NX pte
> 0xffffffff82400000-0xffffffff82a00000 6M ro PSE GLB NX pmd
> 0xffffffff82a00000-0xffffffff82b40000 1280K ro GLB NX pte
> 0xffffffff82b40000-0xffffffff82c00000 768K RW GLB NX pte
> 0xffffffff82c00000-0xffffffff82e00000 2M RW PSE GLB NX pmd
> 0xffffffff82e00000-0xffffffff83000000 2M RW GLB NX pte
> 0xffffffff83000000-0xffffffff83800000 8M RW PSE GLB NX pmd
>
> With the gaps removed/unmapped (pti=on)
>
> Before:
>
> 0xffffffff81000000-0xffffffff81200000 2M ro PSE GLB x pmd
> 0xffffffff81200000-0xffffffff82200000 16M ro PSE x pmd
> 0xffffffff82200000-0xffffffff8231c000 1136K ro x pte
> 0xffffffff8231c000-0xffffffff82400000 912K pte
> 0xffffffff82400000-0xffffffff82a00000 6M ro PSE NX pmd
> 0xffffffff82a00000-0xffffffff82b40000 1280K ro NX pte
> 0xffffffff82b40000-0xffffffff82c00000 768K pte
> 0xffffffff82c00000-0xffffffff83400000 8M RW PSE NX pmd
> 0xffffffff83400000-0xffffffff8342a000 168K RW NX pte
> 0xffffffff8342a000-0xffffffff836f3000 2852K pte
> 0xffffffff836f3000-0xffffffff83800000 1076K RW NX pte
>
> After:
>
> 0xffffffff81000000-0xffffffff81200000 2M ro PSE GLB x pmd
> 0xffffffff81200000-0xffffffff82200000 16M ro PSE x pmd
> 0xffffffff82200000-0xffffffff8231c000 1136K ro x pte
> 0xffffffff8231c000-0xffffffff82400000 912K pte
> 0xffffffff82400000-0xffffffff82a00000 6M ro PSE NX pmd
> 0xffffffff82a00000-0xffffffff82b40000 1280K ro NX pte
> 0xffffffff82b40000-0xffffffff82e3d000 3060K pte
> 0xffffffff82e3d000-0xffffffff83000000 1804K RW NX pte
> 0xffffffff83000000-0xffffffff83800000 8M RW PSE NX pmd
>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> arch/x86/kernel/vmlinux.lds.S | 91 +++++++++++---------
> arch/x86/mm/init_64.c | 5 +-
> arch/x86/mm/pat/set_memory.c | 2 +-
> 3 files changed, 52 insertions(+), 46 deletions(-)

I guess we could do this - I don't see why not... we'll have to take it for
a longer spin tho.

> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 3a24a3fc55f5..1dee2987c42b 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -61,12 +61,15 @@ const_cpu_current_top_of_stack = cpu_current_top_of_stack;
> #define X86_ALIGN_RODATA_BEGIN . = ALIGN(HPAGE_SIZE);
>
> #define X86_ALIGN_RODATA_END \
> - . = ALIGN(HPAGE_SIZE); \
> - __end_rodata_hpage_align = .; \

$ git grep __end_rodata_hpage_align
arch/x86/include/asm/sections.h:13:extern char __end_rodata_hpage_align[];
arch/x86/tools/relocs.c:93: "__end_rodata_hpage_align|"

I guess you wanna remove those too and say that that marker is unused. Better
yet do that in a pre-patch.

> - __end_rodata_aligned = .;
> + . = ALIGN(PAGE_SIZE); \
> + __end_rodata_aligned = ALIGN(HPAGE_SIZE);
>
> #define ALIGN_ENTRY_TEXT_BEGIN . = ALIGN(PMD_SIZE);
> #define ALIGN_ENTRY_TEXT_END . = ALIGN(PMD_SIZE);
> +
> +#define DATA_SEGMENT_START \
> + . = ALIGN(HPAGE_SIZE); \
> + __data_segment_start = .;
> #else
>
> #define X86_ALIGN_RODATA_BEGIN

...

--
Regards/Gruss,
Boris.