On Tue, Feb 10, 2009 at 02:17:49PM -0800, Jeremy Fitzhardinge wrote: > Marcelo Tosatti wrote: >> KVM's paravirt mmu pte batching has issues with, at least, kernel >> updates from DEBUG_PAGEALLOC. >> >> This has been experienced with slab allocation from irq context from >> within lazy mmu sections: >> >> https://bugzilla.redhat.com/show_bug.cgi?id=480822 >> >> DEBUG_PAGEALLOC will map/unmap the kernel pagetables to catch bad >> accesses, with code such as: >> >> __change_page_attr(): >> >> /* >> * Do we really change anything ? >> */ >> if (pte_val(old_pte) != pte_val(new_pte)) { >> set_pte_atomic(kpte, new_pte); >> cpa->flags |= CPA_FLUSHTLB; >> } >> >> A present->nonpresent update can be queued, but not yet committed to >> memory. So the set_pte_atomic will be skipped but the update flushed >> afterwards. set_pte_ATOMIC. >> > > Are you saying that there's a queued update which means that old_pte is > a stale value which happens to equal new_pte, so new_pte is never set? > OK, sounds like a generic problem, of the same sort we've had with > kmap_atomic being used in interrupt routines in lazy mode. Yes. It seems however that only set_pte_at/pte_update/_defer are used under significatly long lazy mmu sections (long as in number of updates). Is it worthwhile to bother (and risk) batching kernel pte updates ? Until someone forgets about arch_flush_lazy_mmu_mode again... > In this case, I think the proper fix is to call > arch_flush_lazy_mmu_mode() before reading old_pte to make sure its up to > date, and calling it again when processing CPA_FLUSHTLB. > Could you try the patch below instead? It should work yes. > (BTW, set_pte_atomic doesn't mean synchronous; it just means its safe to > use on live ptes on 32-bit PAE machines which can't otherwise atomically > update a pte.) Doh, of course. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
