Re: [v3.9-rc8]: kernel BUG at mm/memcontrol.c:3994! (was: Re: [BUG][s390x] mm: system crashed)

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, May 01, 2013 at 08:28:30AM -0700, Hugh Dickins wrote:
> On Tue, 30 Apr 2013, Johannes Weiner wrote:
> > On Wed, Apr 24, 2013 at 08:50:01PM -0700, Hugh Dickins wrote:
> > > On Wed, 24 Apr 2013, Johannes Weiner wrote:
> > > > On Wed, Apr 24, 2013 at 03:18:51PM +0200, Michal Hocko wrote:
> > > > > On Wed 24-04-13 12:42:55, Heiko Carstens wrote:
> > > > > > On Thu, Apr 18, 2013 at 09:13:03AM +0200, Heiko Carstens wrote:
> > > > > > 
> > > > > > [   48.347963] ------------[ cut here ]------------
> > > > > > [   48.347972] kernel BUG at mm/memcontrol.c:3994!
> > > > > > __mem_cgroup_uncharge_common() triggers:
> > > > > > 
> > > > > > [...]
> > > > > >         if (mem_cgroup_disabled())
> > > > > >                 return NULL;
> > > > > > 
> > > > > >         VM_BUG_ON(PageSwapCache(page));
> > > > > > [...]
> > > 
> > > I agree that the actual memcg uncharging should be okay, but the memsw
> > > swap stats will go wrong (doesn't matter toooo much), and mem_cgroup_put
> > > get missed (leaking a struct mem_cgroup).
> > 
> > Ok, so I just went over this again.  For the swapout path the memsw
> > uncharge is deferred, but if we "steal" this uncharge from the swap
> > code, we actually do uncharge memsw in mem_cgroup_do_uncharge(), so we
> > may prematurely unaccount the swap page, but we never leak a charge.
> > Good.
> > 
> > Because of this stealing, we also don't do the following:
> > 
> > 	if (do_swap_account && ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT) {
> > 		mem_cgroup_swap_statistics(memcg, true);
> > 		mem_cgroup_get(memcg);
> > 	}
> > 
> > I.e. it does not matter that mem_cgroup_uncharge_swap() doesn't do the
> > put, we are also not doing the get.  We should not leak references.
> > 
> > So the only thing that I can see go wrong is that we may have a
> > swapped out page that is not charged to memsw and not accounted as
> > MEM_CGROUP_STAT_SWAP.  But I don't know how likely that is, because we
> > check for PG_swapcache in this uncharge path after the last pte is
> > torn down, so even though the page is put on swap cache, it probably
> > won't be swapped.  It would require that the PG_swapcache setting
> > would become visible only after the page has been added to the swap
> > cache AND rmap has established at least one swap pte for us to
> > uncharge a page that actually continues to be used.  And that's a bit
> > of a stretch, I think.
> 
> Sorry, our minds seem to work in different ways,
> I understood very little of what you wrote above :-(
> 
> But once I try to disprove you with a counter-example, I seem to
> arrive at the same conclusion as you have (well, I haven't quite
> arrived there yet, but cannot give it any more time).

I might be losing my mind.  But since you are reaching the same
conclusion, and I see the same mental milestones in your thought
process described below, it's more likely that I suck at describing my
train of thought coherently.  Or the third possibility: we're both
losing it!

> Looking at it from my point of view, I concentrate on the racy
> 	if (PageSwapCache(page))
> 		return;
> 	__mem_cgroup_uncharge_common(page, MEM_CGROUP_CHARGE_TYPE_ANON, false);
> in mem_cgroup_uncharge_page().
> 
> Now, that may or may not catch the case where last reference to page
> is unmapped at the same time as the page is added to swap: but being
> a MEM_CGROUP_CHARGE_TYPE_ANON call, it does not interfere with the
> memsw stats and get/put at all, those remain in balance.

Yes, exactly.

> And mem_cgroup_uncharge_swap() has all along been prepared to get
> a zero id from swap_cgroup_record(), if a SwapCache page should be
> uncharged when it was never quite charged as such.
> 
> Yes, we may occasionally fail to charge a SwapCache page as such
> if its final unmap from userspace races with its being added to swap;
> but it's heading towards swap_writepage()'s try_to_free_swap() anyway,
> so I don't think that's anything to worry about.

Agreed as well.  If there are no pte references to the swap slot, it
will be freed either way.  I didn't even think of the
try_to_free_swap() in the writeout call, but was looking at the
__remove_mapping later on in reclaim that will do a swapcache_free().

The only case I was worried about is the following:

#0                                      #1
page_remove_rmap()                      shrink_page_list()
  if --page->mapcount == 0:               add_to_swap()
    mem_cgroup_uncharge_page()              __add_to_swap_cache()
      if PageSwapCache:                       SetPageSwapCache()
        return                            try_to_unmap()
      __mem_cgroup_uncharge_common()        for each pte:
                                              install swp_entry_t
                                              page->mapcount--

Looking at #1, I don't see anything that would force concurrent
threads to observe SetSwapCache ordered against the page->mapcount--.
My concern was that if those get reordered, #0 may see page->mapcount
== 1 AND !PageSwapcache, and then go ahead and uncharge the page while
there is actually a swp_entry_t pointing to it.  The page will be a
proper long-term swap page without being charged as such.

> (If I had time to stop and read through that, I'd probably find it
> just as hard to understand as what you wrote!)
> 
> > 
> > Did I miss something?  If not, I'll just send a patch that removes the
> > VM_BUG_ON() and adds a comment describing the scenarios and a note
> > that we may want to fix this in the future.
> 
> I don't think you missed something.  Yes, please just send Linus and
> Andrew a patch to remove the VM_BUG_ON() (with Cc stable tag), I now
> agree that's all that's really needed - thanks.

Will do, thanks for taking them time to think through it again, even
after failing to decipher my ramblings...

Johannes

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