Re: CONFIG_VMAP_STACK, on-stack struct, and wake_up_bit

[Andy Lutomirski]

On Wed, Oct 26, 2016 at 5:51 AM, Andreas Gruenbacher
<agruenba@xxxxxxxxxx> wrote:
> Hi,
>
> CONFIG_VMAP_STACK has broken gfs2 and I'm trying to figure out what's
> going on. What I'm seeing is the following: on a fresh gfs2 filesystem
> created with:
>
>   mkfs.gfs2 -p lock_nolock $DEVICE
>
> I get the following BUG with 4.9-rc2, CONFIG_VMAP_STACK and
> CONFIG_DEBUG_VIRTUAL turned on:
>
>   kernel BUG at arch/x86/mm/physaddr.c:26!
>
> Stack of kernel thread:
>
>   __phys_addr(x)
>   bit_waitqueue(word, bit)
>   wake_up_bit(word = &gh->gh_iflags, bit = HIF_WAIT)
>   gfs2_holder_wake(gh)

It's this:

const struct zone *zone = page_zone(virt_to_page(word));

If the stack is vmalloced, then you can't find the page's zone like
that.  We could look it up the slow way (ick!), but maybe another
solution would be to do:

wait_queue_head_t *wait_table;
if (virt_addr_valid(word))
  wait_table = page_zone(virt_to_page(word))->wait_table;
else
  wait_table = funny_wait_table;

where funny_wait_table is an extra wait table just for funny addresses.

This will scale poorly on very large NUMA systems where many zones are
simultaneously using on-stack wait_bit bits, but I suspect this is a
very rare use case.

>
> Is accessing a struct on another kernel thread's stack no longer working?

That part should be fine.