[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] xen.git branch reorg / success with 2.6.30-rc3 pv_ops dom0
On 06/11/09 02:02, Ian Campbell wrote: On Tue, 2009-06-09 at 13:28 -0400, Jeremy Fitzhardinge wrote:Ian Campbell wrote: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 Hmm, no this is about the PagePinned flag on the struct page which is out of date WRT the actual pinned status as Xen sees it -- we update the PagePinned flag early in xen_pin_page() long before Xen the pin hypercall so this window is the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.Yes, it looks like you could get a bad mapping here. An obvious fix would be to defer clearing the pinned flag in the page struct until after the hypercall has issued. That would make the racy kmap_atomic_pte map RO, which would be fine unless it actually tries to modify it (but I can't imagine it would do that unlocked).But would it redo the mapping after taking the lock? It doesn't look like it does (why would it). So we could end up writing to an unpinned pte via a R/O mapping. Hm, yep. One thing I noticed is that set_pte() is used very rarely, so it would be no cost to always use a hypercall in that case. But xen_set_pte_at() ends up calling xen_set_pte() as well, and I think that's more common. Certainly we need to make sure that we're actually taking advantage of late-pin by direct writing unpinned ptes. I've been thinking of rearranging the set_pte(_at) pvops a little bit anyway; its not obvious we're really getting much benefit from using the update_va_mapping hypercall, and if we're not using it, then the set_pte_at pvop is taking a lot of unused parameters. If we switch to just using mmu_update, then we can just pass the address and pte value. But we could also pass the struct page * (which makes a bit of conceptual sense), so we could easy directly test whether the pte is pinned, and either use a direct write or hypercall accordingly. As an experiment I tried the simple approach of flushing the multicalls explicitly in xen_unpin_page and then clearing the Pinned bit and it all goes a bit wrong. eip is "ptep->pte_low = 0" so I think the unpinned but R/O theory holds... Yes, I think the theory is sound. But I'm curious why Pasi seems to be able to hit the race easily, but we have not... J _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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