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

[Dmitry Vyukov]

On Fri, Feb 10, 2017 at 10:54 AM, Andrey Ryabinin
<aryabinin@xxxxxxxxxxxxx> wrote:
> CONFIG_KASAN=y needs a lot of virtual memory mapped for its shadow.
> In that case ptdump_walk_pgd_level_core() takes a lot of time to
> walk across all page tables and doing this without
> a rescheduling causes soft lockups:
>
>  NMI watchdog: BUG: soft lockup - CPU#3 stuck for 23s! [swapper/0:1]
>  ...
>  Call Trace:
>   ptdump_walk_pgd_level_core+0x40c/0x550
>   ptdump_walk_pgd_level_checkwx+0x17/0x20
>   mark_rodata_ro+0x13b/0x150
>   kernel_init+0x2f/0x120
>   ret_from_fork+0x2c/0x40
>
> I guess that this issue might arise even without KASAN on a huge
> machines with several terabytes of RAM.
>
> Stick cond_resched() in pgd loop to fix this.
>
> Reported-by: Tobias Regnery <tobias.regnery@xxxxxxxxx>
> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/mm/dump_pagetables.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
> index ea9c49a..8aa6bea 100644
> --- a/arch/x86/mm/dump_pagetables.c
> +++ b/arch/x86/mm/dump_pagetables.c
> @@ -15,6 +15,7 @@
>  #include <linux/debugfs.h>
>  #include <linux/mm.h>
>  #include <linux/init.h>
> +#include <linux/sched.h>
>  #include <linux/seq_file.h>
>
>  #include <asm/pgtable.h>
> @@ -406,6 +407,7 @@ static void ptdump_walk_pgd_level_core(struct seq_file *m, pgd_t *pgd,
>                 } else
>                         note_page(m, &st, __pgprot(0), 1);
>
> +               cond_resched();


This is the right thing to do per se, but I am concerned that now
people will just suffers from slow boot (it can take literally
minutes) and will not realize the root cause nor that it's fixable
(e.g. with rodata=n) and will probably just blame KASAN for slowness.

Could we default this rodata check to n under KASAN? Or at least print
some explanatory warning message before doing marking rodata (it
should be printed right before "hang", so if you stare at it for a
minute during each boot you realize that it may be related)? Or
something along these lines. FWIW in my builds I just always disable
the check.