Re: [Xen-devel] xen.git branch reorg / success with 2.6.30-rc3 pv_ops dom0

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Mon, Jun 08, 2009 at 08:05:43PM +0300, Pasi Kärkkäinen wrote:
> On Mon, Jun 08, 2009 at 07:21:46PM +0300, Pasi Kärkkäinen wrote:
> > On Mon, Jun 08, 2009 at 05:17:45PM +0100, Ian Campbell wrote:
> > > On Mon, 2009-06-08 at 12:13 -0400, Pasi Kärkkäinen wrote:
> > > > On Mon, Jun 08, 2009 at 05:00:58PM +0100, Ian Campbell wrote:
> > > > > On Mon, 2009-06-08 at 11:45 -0400, Ian Campbell wrote:
> > > > > > 
> > > > > > > L4 at e1822000 is pinned contains L2 at e1977228 which points at 
> > > > > > > an
> > > > > > L1
> > > > > > > which is unpinned low mem address 0x8bf8000
> > > > > > 
> > > > > > OK so I think that is interesting. A pinned L4 referencing an 
> > > > > > unpinned
> > > > > > L1 isn't supposed to happen, I don't think (Jeremy?).
> > > > > 
> > > > > Interesting:
> > > > > 
> > > > >         pte_t *page_check_address(struct page *page, struct mm_struct 
> > > > > *mm,
> > > > >         [...]
> > > > >               pte = pte_offset_map(pmd, address); /* A */
> > > > >               /* Make a quick check before getting the lock */
> > > > >               if (!sync && !pte_present(*pte)) {
> > > > >                       pte_unmap(pte);
> > > > >                       return NULL;
> > > > >               }
> > > > >         
> > > > >               ptl = pte_lockptr(mm, pmd);
> > > > >               spin_lock(ptl);
> > > > >         [...]
> > > > >         
> > > > > So at point A we make a new mapping of a PTE without yet holding the
> > > > > corresponding PTE lock and this is precisely the point at which things
> > > > > start to go wrong for us... (coincidence? I think not ;-))
> > > > > 
> > > > > I wonder how this interacts with the logic in
> > > > > arch/x86/xen/mmu.c:xen_pin_page() which holds the lock while waiting 
> > > > > for
> > > > > the (deferred) pin multicall to occur? Hmm, no this is about the
> > > > > PagePinned flag on the struct page which is out of date WRT the actual
> > > > > pinned status as Xen sees it -- we update the PagePinned flag early in
> > > > > xen_pin_page() long before Xen the pin hypercall so this window is the
> > > > > other way round to what would be needed to trigger this bug.
> > > > > 
> > > > > On the other hand xen_unpin_page() looks like it sets up something
> > > > > roughly like what we need for this issue to trigger.
> > > > > 
> > > > > Pasi in additional to my other mad hack could you try this:
> > > > > 
> > > > 
> > > > Ok.. do you want me to try first without this patch? Or should I cancel 
> > > > my
> > > > kernel compilation and apply this aswell? :)
> > > 
> > > Can you try the first patch first then add this one please.
> > > 
> > 
> > Ok. Will do.
> > 
> > I was already starting to feel like 'maybe my hardware is broken' but now 
> > that
> > code looks like it might be an actual bug :)
> > 
> > Let's see.
> > 
> 
> Crash with only the first patch applied:
> http://pasik.reaktio.net/xen/pv_ops-dom0-debug/pv_ops-dom0-log-05-with-highpte-no-swap-with-debug3.txt
> 
> Now I'll try with the second one included aswell..
> 

And here's one with the second patch applied aswell:
http://pasik.reaktio.net/xen/pv_ops-dom0-debug/pv_ops-dom0-log-06-with-highpte-no-swap-with-debug4.txt

Seems to be different.. Xen is not complaining anymore..

-- Pasi

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