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

[Kirill A. Shutemov]

On Mon, Jul 24, 2017 at 06:57:53PM +0300, Andrey Ryabinin wrote:
> 
> 
> On 07/24/2017 06:37 PM, Kirill A. Shutemov wrote:
> > 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@xxxxxxxxxxxxx>
> >> ---
> >>  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?
> > 
> 
> No, 5 is correct, because all kasan page tables endup as a pte mapping of kasan_zero_page.
> And pte is level 5.
> 
> Anything but 5 would give us incorrect output in the last column in the last column of the dump,
> pmd,pud or p4d instead of correct pte.
> 
> Without the patch, if we stump on kasan page table we always endup with a note_page(..., 5) call.
> With this patch we just avoid useless walks on lower levels of page tables, because we already know the end result,
> so we just call note_page(.., 5) right away.

You're right. Thanks for explanation.

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov