On Tue, Feb 14, 2017 at 11:08 AM, Andrey Ryabinin <[email protected]> wrote: > Enabling both DEBUG_WX=y and KASAN=y options significantly increases > boot time (dozens of seconds at least). > KASAN fills kernel page tables with repeated values to map several > TBs of the virtual memory to the single kasan_zero_page: > > kasan_zero_pud -> > kasan_zero_pmd-> > kasan_zero_pte-> > kasan_zero_page > > So, the page table walker used to find W+X mapping check the same > kasan_zero_p?d page table entries a lot more than once. > With patch pud walker will skip the pud if it has the same value as > the previous one . Skipping done iff we search for W+X mappings, > so this optimization won't affect the page table dump via debugfs. > > This dropped time spend in W+X check from ~30 sec to reasonable 0.1 sec: > > Before: > [ 4.579991] Freeing unused kernel memory: 1000K > [ 35.257523] x86/mm: Checked W+X mappings: passed, no W+X pages found. > > After: > [ 5.138756] Freeing unused kernel memory: 1000K > [ 5.266496] x86/mm: Checked W+X mappings: passed, no W+X pages found. > > Signed-off-by: Andrey Ryabinin <[email protected]> Reviewed-by: Alexander Potapenko <[email protected]> > --- > arch/x86/mm/dump_pagetables.c | 16 +++++++++++++++- > 1 file changed, 15 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c > index 8aa6bea..0813534 100644 > --- a/arch/x86/mm/dump_pagetables.c > +++ b/arch/x86/mm/dump_pagetables.c > @@ -327,18 +327,31 @@ static void walk_pmd_level(struct seq_file *m, struct > pg_state *st, pud_t addr, > > #if PTRS_PER_PUD > 1 > > +/* > + * This is an optimization for CONFIG_DEBUG_WX=y + CONFIG_KASAN=y > + * KASAN fills page tables with the same values. Since there is no > + * point in checking page table more than once we just skip repeated > + * entries. This saves us dozens of seconds during boot. > + */ > +static bool pud_already_checked(pud_t *prev_pud, pud_t *pud, bool checkwx) > +{ > + return checkwx && prev_pud && (pud_val(*prev_pud) == pud_val(*pud)); > +} > + > static void walk_pud_level(struct seq_file *m, struct pg_state *st, pgd_t > addr, > unsigned long P) > { > int i; > pud_t *start; > pgprotval_t prot; > + pud_t *prev_pud = NULL; > > start = (pud_t *) pgd_page_vaddr(addr); > > for (i = 0; i < PTRS_PER_PUD; i++) { > st->current_address = normalize_addr(P + i * PUD_LEVEL_MULT); > - if (!pud_none(*start)) { > + if (!pud_none(*start) && > + !pud_already_checked(prev_pud, start, st->check_wx)) { > if (pud_large(*start) || !pud_present(*start)) { > prot = pud_flags(*start); > note_page(m, st, __pgprot(prot), 2); > @@ -349,6 +362,7 @@ static void walk_pud_level(struct seq_file *m, struct > pg_state *st, pgd_t addr, > } else > note_page(m, st, __pgprot(0), 2); > > + prev_pud = start; > start++; > } > } > -- > 2.10.2 >
-- Alexander Potapenko Software Engineer Google Germany GmbH Erika-Mann-Straße, 33 80636 München Geschäftsführer: Matthew Scott Sucherman, Paul Terence Manicle Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg
