[PATCH v2 3/4] arm64: mm: make vmemmap region a projection of the linear region

[Geert Uytterhoeven]

Hi Ard,

On Thu, Oct 8, 2020 at 5:43 PM Ard Biesheuvel <ardb at kernel.org> wrote:
> Now that we have reverted the introduction of the vmemmap struct page
> pointer and the separate physvirt_offset, we can simplify things further,
> and place the vmemmap region in the VA space in such a way that virtual
> to page translations and vice versa can be implemented using a single
> arithmetic shift.
>
> One happy coincidence resulting from this is that the 48-bit/4k and
> 52-bit/64k configurations (which are assumed to be the two most
> prevalent) end up with the same placement of the vmemmap region. In
> a subsequent patch, we will take advantage of this, and unify the
> memory maps even more.
>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>

This is now commit 8c96400d6a39be76 ("arm64: mm: make vmemmap region a
projection of the linear region") in arm64/for-next/core.

> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -504,6 +504,8 @@ static void __init free_unused_memmap(void)
>   */
>  void __init mem_init(void)
>  {
> +       BUILD_BUG_ON(!is_power_of_2(sizeof(struct page)));

This check is triggering for me.

If CONFIG_MEMCG=n, sizeof(struct page) = 56.

If CONFIG_HAVE_ALIGNED_STRUCT_PAGE=y, this is mitigated by
the explicit alignment:

    #ifdef CONFIG_HAVE_ALIGNED_STRUCT_PAGE
    #define _struct_page_alignment  __aligned(2 * sizeof(unsigned long))
    #else
    #define _struct_page_alignment
    #endif

    struct page { ... } _struct_page_alignment;

However, HAVE_ALIGNED_STRUCT_PAGE is selected only if SLUB,
while my .config is using SLAB.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds