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 <aryabi...@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
