linux-kernel - Re: [PATCH -v2 08/10] m68k,mm: Extend table allocator for multiple sizes

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Message-ID: <CAMuHMdWa8R=3fHLV7W_ni8An_1CwOoJxErnnDA3t4rq2XN+QzA@mail.gmail.com>
Date:   Fri, 7 Feb 2020 11:56:40 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        Michael Schmitz <schmitzmic@...il.com>,
        Greg Ungerer <gerg@...ux-m68k.org>
Subject: Re: [PATCH -v2 08/10] m68k,mm: Extend table allocator for multiple sizes

Hoi Peter,

On Fri, Jan 31, 2020 at 1:56 PM Peter Zijlstra <peterz@...radead.org> wrote:
> In addition to the PGD/PMD table size (128*4) add a PTE table size
> (64*4) to the table allocator. This completely removes the pte-table
> overhead compared to the old code, even for dense tables.

Thanks for your patch!

> Notes:
>
>  - the allocator gained a list_empty() check to deal with there not
>    being any pages at all.
>
>  - the free mask is extended to cover more than the 8 bits required
>    for the (512 byte) PGD/PMD tables.

Being an mm-illiterate, I don't understand the relation between the number
of bits and the size (see below).

>  - NR_PAGETABLE accounting is restored.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

WARNING: Missing Signed-off-by: line by nominal patch author 'Peter
Zijlstra <peterz@...radead.org>'
(in all patches)

I can fix that (the From?) up while applying.

> --- a/arch/m68k/mm/motorola.c
> +++ b/arch/m68k/mm/motorola.c
> @@ -72,24 +72,35 @@ void mmu_page_dtor(void *page)
>     arch/sparc/mm/srmmu.c ... */
>
>  typedef struct list_head ptable_desc;
> -static LIST_HEAD(ptable_list);
> +
> +static struct list_head ptable_list[2] = {
> +       LIST_HEAD_INIT(ptable_list[0]),
> +       LIST_HEAD_INIT(ptable_list[1]),
> +};
>
>  #define PD_PTABLE(page) ((ptable_desc *)&(virt_to_page(page)->lru))
>  #define PD_PAGE(ptable) (list_entry(ptable, struct page, lru))
> -#define PD_MARKBITS(dp) (*(unsigned char *)&PD_PAGE(dp)->index)
> +#define PD_MARKBITS(dp) (*(unsigned int *)&PD_PAGE(dp)->index)
> +
> +static const int ptable_shift[2] = {
> +       7+2, /* PGD, PMD */
> +       6+2, /* PTE */
> +};
>
> -#define PTABLE_SIZE (PTRS_PER_PMD * sizeof(pmd_t))
> +#define ptable_size(type) (1U << ptable_shift[type])
> +#define ptable_mask(type) ((1U << (PAGE_SIZE / ptable_size(type))) - 1)

So this is 0xff for PGD and PMD, like before, and 0xffff for PTE.
Why the latter value?

Thanks!

>
> -void __init init_pointer_table(unsigned long ptable)
> +void __init init_pointer_table(void *table, int type)
>  {
>         ptable_desc *dp;
> +       unsigned long ptable = (unsigned long)table;
>         unsigned long page = ptable & PAGE_MASK;
> -       unsigned char mask = 1 << ((ptable - page)/PTABLE_SIZE);
> +       unsigned int mask = 1U << ((ptable - page)/ptable_size(type));
>
>         dp = PD_PTABLE(page);
>         if (!(PD_MARKBITS(dp) & mask)) {
> -               PD_MARKBITS(dp) = 0xff;
> -               list_add(dp, &ptable_list);
> +               PD_MARKBITS(dp) = ptable_mask(type);
> +               list_add(dp, &ptable_list[type]);
>         }
>
>         PD_MARKBITS(dp) &= ~mask;

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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