linux-kernel - Re: [PATCH 4/5] m68k,mm: Extend table allocator for multiple sizes

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Date:   Wed, 29 Jan 2020 12:17:53 +0000
From:   Will Deacon <will@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k@...ts.linux-m68k.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] m68k,mm: Extend table allocator for multiple sizes

On Wed, Jan 29, 2020 at 11:39:45AM +0100, Peter Zijlstra 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.
> 
> Notes:
> 
>  - the allocator gained __flush_page_to_ram(), since the old
>    page-based allocator had that.
> 
>  - 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.
> 
>  - NR_PAGETABLE accounting is restored.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/m68k/include/asm/motorola_pgalloc.h |   24 +++++-----
>  arch/m68k/mm/init.c                      |    6 +-
>  arch/m68k/mm/memory.c                    |   70 ++++++++++++++++++++-----------
>  3 files changed, 61 insertions(+), 39 deletions(-)
> 
> --- a/arch/m68k/include/asm/motorola_pgalloc.h
> +++ b/arch/m68k/include/asm/motorola_pgalloc.h
> @@ -5,61 +5,61 @@
>  #include <asm/tlb.h>
>  #include <asm/tlbflush.h>
>  
> -extern pmd_t *get_pointer_table(void);
> -extern int free_pointer_table(pmd_t *);
> +extern void *get_pointer_table(int type);

Could be prettier/obfuscated with an enum type?

> --- a/arch/m68k/mm/memory.c
> +++ b/arch/m68k/mm/memory.c
> @@ -27,24 +27,34 @@
>     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)
>  
> -void __init init_pointer_table(unsigned long ptable)
> +void __init init_pointer_table(unsigned long ptable, int type)
>  {
>  	ptable_desc *dp;
>  	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;
> @@ -57,12 +67,10 @@ void __init init_pointer_table(unsigned
>  	return;
>  }
>  
> -pmd_t *get_pointer_table (void)
> +void *get_pointer_table (int type)
>  {
> -	ptable_desc *dp = ptable_list.next;
> -	unsigned char mask = PD_MARKBITS (dp);
> -	unsigned char tmp;
> -	unsigned int off;
> +	ptable_desc *dp = ptable_list[type].next;
> +	unsigned int mask, tmp, off;

nit, but if you do:

	unsigned int mask = list_empty(&ptable_list[type]) ? 0 : PD_MARKBITS(dp);

then you can leave the existing mask logic as-is.

Will