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

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Date:   Fri, 10 Feb 2017 14:41:47 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Tobias Regnery <tobias.regnery@...il.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Alexander Potapenko <glider@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] x86/mm/ptdump: Fix soft lockup in page table walker.

On Fri, Feb 10, 2017 at 05:38:20PM +0300, Andrey Ryabinin wrote:
> On 02/10/2017 05:29 PM, Mark Rutland wrote:
> > 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@...tuozzo.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.
> > 
> 
> For userspace pagetable dumps checkwx is false, so
> page_already_checked() will return false and will not go into else.
> userspace pagetable dumps works as before.

Ah. I missed that; sorry for the noise.

That sounds ok then, though it's probably worth a comment as to what
we're doing this for.

Thanks,
Mark.