linux-kernel - Re: [PATCH 3/5] m68k,mm: Use table allocator for pgtables

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Message-ID: <20200129121149.GA31582@willie-the-truck>
Date:   Wed, 29 Jan 2020 12:11:50 +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 3/5] m68k,mm: Use table allocator for pgtables

On Wed, Jan 29, 2020 at 11:39:44AM +0100, Peter Zijlstra wrote:
> With the new page-table layout, using full (4k) pages for (256 byte)
> pte-tables is immensely wastefull. Move the pte-tables over to the
> same allocator already used for the (512 byte) higher level tables
> (pgd/pmd).
> 
> This reduces the pte-table waste from 15x to 2x.
> 
> Due to no longer being bound to 16 consecutive tables, this might
> actually already be more efficient than the old code for sparse
> tables.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/m68k/include/asm/motorola_pgalloc.h |   54 ++++++-------------------------
>  arch/m68k/include/asm/motorola_pgtable.h |    8 ++++
>  arch/m68k/include/asm/page.h             |    2 -
>  3 files changed, 19 insertions(+), 45 deletions(-)
> 
> --- a/arch/m68k/include/asm/motorola_pgalloc.h
> +++ b/arch/m68k/include/asm/motorola_pgalloc.h
> @@ -10,60 +10,28 @@ extern int free_pointer_table(pmd_t *);
>  
>  static inline pte_t *pte_alloc_one_kernel(struct mm_struct *mm)
>  {
> -	pte_t *pte;
> -
> -	pte = (pte_t *)__get_free_page(GFP_KERNEL|__GFP_ZERO);
> -	if (pte) {
> -		__flush_page_to_ram(pte);
> -		flush_tlb_kernel_page(pte);
> -		nocache_page(pte);
> -	}
> -
> -	return pte;
> +	return (pte_t *)get_pointer_table();

Weirdly, get_pointer_table() seems to elide the __flush_page_to_ram()
call, so you're missing that for ptes with this change. I think it's
probably needed for the higher levels too (and kernel_page_table()
does it for example) so I'd be inclined to add it unconditionally
rather than predicate it on the allocation type introduced by your later
patch.

> --- a/arch/m68k/include/asm/page.h
> +++ b/arch/m68k/include/asm/page.h
> @@ -30,7 +30,7 @@ typedef struct { unsigned long pmd; } pm
>  typedef struct { unsigned long pte; } pte_t;
>  typedef struct { unsigned long pgd; } pgd_t;
>  typedef struct { unsigned long pgprot; } pgprot_t;
> -typedef struct page *pgtable_t;
> +typedef pte_t *pgtable_t;

Urgh, this is a big (cross-arch) mess that we should fix later.

Will