Re: mm: kernel BUG at mm/huge_memory.c:2785!

[Sasha Levin]

On 03/07/2014 07:18 AM, Kirill A. Shutemov wrote:
> On Thu, Mar 06, 2014 at 08:32:48PM -0500, Sasha Levin wrote:
> > On 03/04/2014 10:53 PM, Sasha Levin wrote:
> > > On 03/04/2014 10:16 PM, Bob Liu wrote:
> > > > On Thu, Feb 27, 2014 at 11:03 PM, Kirill A. Shutemov
> > > > <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > > > > Sasha Levin wrote:
> > > > > > Hi all,
> > > > > >
> > > > > > While fuzzing with trinity inside a KVM tools guest running latest -next kernel I've stumbled on the
> > > > > > following spew:
> > > > > >
> > > > > > [ 1428.146261] kernel BUG at mm/huge_memory.c:2785!
> > > > >
> > > > > Hm, interesting.
> > > > >
> > > > > It seems we either failed to split huge page on vma split or it
> > > > > materialized from under us. I don't see how it can happen:
> > > > >
> > > > >    - it seems we do the right thing with vma_adjust_trans_huge() in
> > > > >      __split_vma();
> > > > >    - we hold ->mmap_sem all the way from vm_munmap(). At least I don't see
> > > > >      a place where we could drop it;
> > > >
> > > > Enable CONFIG_DEBUG_VM may show some useful information, at least we
> > > > can confirm weather rwsem_is_locked(&tlb->mm->mmap_sem) before
> > > > split_huge_page_pmd().
> > >
> > > I have CONFIG_DEBUG_VM enabled and that code you're talking is not triggering, so mmap_sem
> > > is locked.
> >
> > Guess what. I've just hit it.
>
> I think this particular traceback is not a real problem: by time of
> exit_mm() we shouldn't race with anybody for the mm_struct.
>
> We probably could drop ->mmap_sem later in mmput() rather then in
> exit_mm() to fix this false positive.
>
> > It's worth keeping in mind that this is the first time I see it.
>
> Hm. That's strange exit_mmap() is called without holding ->mmap_sem.

This issues does happen quite often and is very easy to reproduce, I could try
anything you can thing of.


Thanks,
Sasha
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