Re: [PATCH] x86/mm/dump_pagetables: Speed up page tables dump for CONFIG_KASAN=y

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, Jul 24, 2017 at 06:25:58PM +0300, Andrey Ryabinin wrote:
> KASAN fills kernel page tables with repeated values to map several
> TBs of the virtual memory to the single kasan_zero_page:
>   kasan_zero_p4d ->
>     kasan_zero_pud ->
>         kasan_zero_pmd->
>             kasan_zero_pte->
>                 kasan_zero_page
> 
> Walking the whole KASAN shadow range takes a lot of time, especially
> with 5-level page tables. Since we already know that all kasan page tables
> eventually point to the kasan_zero_page we could call note_page()
> right and avoid walking lower levels of the page tables.
> This will not affect the output of the kernel_page_tables file,
> but let us avoid spending time in page table walkers:
> 
> Before:
> time cat /sys/kernel/debug/kernel_page_tables > /dev/null
> 
> real    0m55.855s
> user    0m0.000s
> sys     0m55.840s
> 
> After:
> time cat /sys/kernel/debug/kernel_page_tables > /dev/null
> 
> real    0m0.054s
> user    0m0.000s
> sys     0m0.054s
> 
> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
> ---
>  arch/x86/mm/dump_pagetables.c | 64 +++++++++++++++++++++++++++----------------
>  1 file changed, 41 insertions(+), 23 deletions(-)
> 
> diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
> index b371ab68f2d4..5e3ac6fe6c9e 100644
> --- a/arch/x86/mm/dump_pagetables.c
> +++ b/arch/x86/mm/dump_pagetables.c
> @@ -13,12 +13,12 @@
>   */
>  
>  #include <linux/debugfs.h>
> +#include <linux/kasan.h>
>  #include <linux/mm.h>
>  #include <linux/init.h>
>  #include <linux/sched.h>
>  #include <linux/seq_file.h>
>  
> -#include <asm/kasan.h>
>  #include <asm/pgtable.h>
>  
>  /*
> @@ -302,23 +302,53 @@ static void walk_pte_level(struct seq_file *m, struct pg_state *st, pmd_t addr,
>  		start++;
>  	}
>  }
> +#ifdef CONFIG_KASAN
> +
> +/*
> + * This is an optimization for KASAN=y case. Since all kasan page tables
> + * eventually point to the kasan_zero_page we could call note_page()
> + * right away without walking through lower level page tables. This saves
> + * us dozens of seconds (minutes for 5-level config) while checking for
> + * W+X mapping or reading kernel_page_tables debugfs file.
> + */
> +static inline bool kasan_page_table(struct seq_file *m, struct pg_state *st,
> +				void *pt)
> +{
> +	if (__pa(pt) == __pa(kasan_zero_pmd) ||
> +#ifdef CONFIG_X86_5LEVEL
> +	    __pa(pt) == __pa(kasan_zero_p4d) ||
> +#endif
> +	    __pa(pt) == __pa(kasan_zero_pud)) {
> +		pgprotval_t prot = pte_flags(kasan_zero_pte[0]);
> +		note_page(m, st, __pgprot(prot), 5);

Hm. I don't think '5' is correct here. Shouldn't it be dependent on what
page table we detected? Or just take it from caller?


-- 
 Kirill A. Shutemov