Re: PROBLEM: Power9: kernel oops on memory hotunplug from ppc64le guest

[Aneesh Kumar K.V]

On 5/20/19 8:25 PM, Nicholas Piggin wrote:
> Bharata B Rao's on May 21, 2019 12:29 am:
> > On Mon, May 20, 2019 at 01:50:35PM +0530, Bharata B Rao wrote:
> > > On Mon, May 20, 2019 at 05:00:21PM +1000, Nicholas Piggin wrote:
> > > > Bharata B Rao's on May 20, 2019 3:56 pm:
> > > > > On Mon, May 20, 2019 at 02:48:35PM +1000, Nicholas Piggin wrote:
> > > > > > > > > git bisect points to
> > > > > > > > >
> > > > > > > > > commit 4231aba000f5a4583dd9f67057aadb68c3eca99d
> > > > > > > > > Author: Nicholas Piggin <npiggin@xxxxxxxxx>
> > > > > > > > > Date:   Fri Jul 27 21:48:17 2018 +1000
> > > > > > > > >
> > > > > > > > >      powerpc/64s: Fix page table fragment refcount race vs speculative references
> > > > > > > > >
> > > > > > > > >      The page table fragment allocator uses the main page refcount racily
> > > > > > > > >      with respect to speculative references. A customer observed a BUG due
> > > > > > > > >      to page table page refcount underflow in the fragment allocator. This
> > > > > > > > >      can be caused by the fragment allocator set_page_count stomping on a
> > > > > > > > >      speculative reference, and then the speculative failure handler
> > > > > > > > >      decrements the new reference, and the underflow eventually pops when
> > > > > > > > >      the page tables are freed.
> > > > > > > > >
> > > > > > > > >      Fix this by using a dedicated field in the struct page for the page
> > > > > > > > >      table fragment allocator.
> > > > > > > > >
> > > > > > > > >      Fixes: 5c1f6ee9a31c ("powerpc: Reduce PTE table memory wastage")
> > > > > > > > >      Cc: stable@xxxxxxxxxxxxxxx # v3.10+
> > > > > > > >
> > > > > > > > That's the commit that added the BUG_ON(), so prior to that you won't
> > > > > > > > see the crash.
> > > > > > >
> > > > > > > Right, but the commit says it fixes page table page refcount underflow by
> > > > > > > introducing a new field &page->pt_frag_refcount. Now we are hitting the underflow
> > > > > > > for this pt_frag_refcount.
> > > > > >
> > > > > > The fixed underflow is caused by a bug (race on page count) that got
> > > > > > fixed by that patch. You are hitting a different underflow here. It's
> > > > > > not certain my patch caused it, I'm just trying to reproduce now.
> > > > >
> > > > > Ok.
> > > >
> > > > Can't reproduce I'm afraid, tried adding and removing 8GB memory from a
> > > > 4GB guest (via host adding / removing memory device), and it just works.
> > >
> > > Boot, add 8G, reboot, remove 8G is the sequence to reproduce.
> > >
> > > > It's likely to be an edge case like an off by one or rounding error
> > > > that just happens to trigger in your config. Might be easiest if you
> > > > could test with a debug patch.
> > >
> > > Sure, I will continue debugging.
> >
> > When the guest is rebooted after hotplug, the entire memory (which includes
> > the hotplugged memory) gets remapped again freshly. However at this time
> > since no slab is available yet, pt_frag_refcount never gets initialized as we
> > never do pte_fragment_alloc() for these mappings. So we right away hit the
> > underflow during the first unplug itself, it looks like.
>
> Nice catch, good debugging work.
>
> > I will check how this can be fixed.
>
> Tricky problem. What do you think? You might be able to make the early
> page table allocations in the same pattern as the frag allocations, and
> then fill in the struct page metadata when you have those.


I guess we need to do something similar to what x86 does. We need to 
walk the init_mm page table again and re-init struct page and other data 
structures backing the tables?

-aneesh