Re: [PATCH] x86/mm/ptdump: Fix soft lockup in page table walker.

[Mark Rutland <mark.rutland@arm.com>]

Hi,

On Fri, Feb 10, 2017 at 04:56:19PM +0300, Andrey Ryabinin wrote:
> On 02/10/2017 04:02 PM, Dmitry Vyukov wrote:
> > On Fri, Feb 10, 2017 at 1:15 PM, Andrey Ryabinin
> > <aryabinin@virtuozzo.com> wrote:
> >> On 02/10/2017 02:18 PM, Thomas Gleixner wrote:
> >>> On Fri, 10 Feb 2017, Dmitry Vyukov wrote:

> diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
> index 8aa6bea..1599a5c 100644
> --- a/arch/x86/mm/dump_pagetables.c
> +++ b/arch/x86/mm/dump_pagetables.c
> @@ -373,6 +373,11 @@ static inline bool is_hypervisor_range(int idx)
>  #endif
>  }
>  
> +static bool pgd_already_checked(pgd_t *prev_pgd, pgd_t *pgd, bool checkwx)
> +{
> +       return checkwx && prev_pgd && (pgd_val(*prev_pgd) == pgd_val(*pgd));
> +}
> +
>  static void ptdump_walk_pgd_level_core(struct seq_file *m, pgd_t *pgd,
>                                        bool checkwx)
>  {
> @@ -381,6 +386,7 @@ static void ptdump_walk_pgd_level_core(struct seq_file *m, pgd_t *pgd,
>  #else
>         pgd_t *start = swapper_pg_dir;
>  #endif
> +       pgd_t *prev_pgd = NULL;
>         pgprotval_t prot;
>         int i;
>         struct pg_state st = {};
> @@ -396,7 +402,8 @@ static void ptdump_walk_pgd_level_core(struct seq_file *m, pgd_t *pgd,
>  
>         for (i = 0; i < PTRS_PER_PGD; i++) {
>                 st.current_address = normalize_addr(i * PGD_LEVEL_MULT);
> -               if (!pgd_none(*start) && !is_hypervisor_range(i)) {
> +               if (!pgd_none(*start) && !is_hypervisor_range(i) &&
> +                               !pgd_already_checked(prev_pgd, start, checkwx)) {

This means we'll fall into the else case...

>                         if (pgd_large(*start) || !pgd_present(*start)) {
>                                 prot = pgd_flags(*start);
>                                 note_page(m, &st, __pgprot(prot), 1);
> @@ -408,6 +415,7 @@ static void ptdump_walk_pgd_level_core(struct seq_file *m, pgd_t *pgd,
>                         note_page(m, &st, __pgprot(0), 1);

... i.e. the note_page() here, where we'll claim that the's nothing
present due to the empty prot.

That'll give erroneous output for the userspace pagetable dumps, so I do
not think this is quite right, even though it gives a boot-time speedup.

Thanks,
Mark.