Hi Zheng, On 12/03/2019 15:30, Zheng Xiang wrote: > Hi Marc, > > On 2019/3/12 19:32, Marc Zyngier wrote: >> Hi Zheng, >> >> On 11/03/2019 16:31, Zheng Xiang wrote: >>> Hi all, >>> >>> While a page is merged into a transparent huge page, KVM will invalidate Stage-2 for >>> the base address of the huge page and the whole of Stage-1. >>> However, this just only invalidates the first page within the huge page and the other >>> pages are not invalidated, see bellow: >>> >>> +---------------+--------------+ >>> |abcde 2MB-Page | >>> +---------------+--------------+ >>> >>> TLB before setting new pmd: >>> +---------------+--------------+ >>> | VA | PAGESIZE | >>> +---------------+--------------+ >>> | a | 4KB | >>> +---------------+--------------+ >>> | b | 4KB | >>> +---------------+--------------+ >>> | c | 4KB | >>> +---------------+--------------+ >>> | d | 4KB | >>> +---------------+--------------+ >>> >>> TLB after setting new pmd: >>> +---------------+--------------+ >>> | VA | PAGESIZE | >>> +---------------+--------------+ >>> | a | 2MB | >>> +---------------+--------------+ >>> | b | 4KB | >>> +---------------+--------------+ >>> | c | 4KB | >>> +---------------+--------------+ >>> | d | 4KB | >>> +---------------+--------------+ >>> >>> When VM access *b* address, it will hit the TLB and result in TLB conflict aborts or other potential exceptions. >> >> That's really bad. I can only imagine two scenarios: >> >> 1) We fail to unmap a,b,c,d (and potentially another 508 PTEs), loosing >> the PTE table in the process, and place the PMD instead. I can't see >> this happening. >> >> 2) We fail to invalidate on unmap, and that slightly less bad (but still >> quite bad). >> >> Which of the two cases are you seeing? >> >>> For example, we need to keep tracking of the VM memory dirty pages when VM is in live migration. >>> KVM will set the memslot READONLY and split the huge pages. >>> After live migration is canceled and abort, the pages will be merged into THP. >>> The later access to these pages which are READONLY will cause level-3 Permission Fault until they are invalidated. >>> >>> So should we invalidate the tlb entries for all relative pages(e.g a,b,c,d), like __flush_tlb_range()? >>> Or we can call __kvm_tlb_flush_vmid() to invalidate all tlb entries. >> >> We should perform an invalidate on each unmap. unmap_stage2_range seems >> to do the right thing. __flush_tlb_range only caters for Stage1 >> mappings, and __kvm_tlb_flush_vmid() is too big a hammer, as it nukes >> TLBs for the whole VM. >> >> I'd really like to understand what you're seeing, and how to reproduce >> it. Do you have a minimal example I could run on my own HW? > > When I start the live migration for a VM, qemu then begins to log and count dirty pages. > During the live migration, KVM set the pages READONLY so that we can count how many pages > would be wrote afterwards. > > Anything is OK until I cancel the live migration and qemu stop logging. Later the VM gets hang. > The trace log shows repeatedly level-3 permission fault caused by a write on a same IPA. After > analyzing the source code, I find KVM always return from the bellow *if* statement in > stage2_set_pmd_huge() even if we only have a single VCPU: > > /* > * Multiple vcpus faulting on the same PMD entry, can > * lead to them sequentially updating the PMD with the > * same value. Following the break-before-make > * (pmd_clear() followed by tlb_flush()) process can > * hinder forward progress due to refaults generated > * on missing translations. > * > * Skip updating the page table if the entry is > * unchanged. > */ > if (pmd_val(old_pmd) == pmd_val(*new_pmd)) > return 0; > > The PMD has already set the PMD_S2_RDWR bit. I doubt kvm_tlb_flush_vmid_ipa() does not invalidate > Stage-2 for the subpages of the PMD(except the first PTE of this PMD). Finally I add some debug > code to flush tlb for all subpages of the PMD, as shown bellow: > > /* > * Mapping in huge pages should only happen through a > * fault. If a page is merged into a transparent huge > * page, the individual subpages of that huge page > * should be unmapped through MMU notifiers before we > * get here. > * > * Merging of CompoundPages is not supported; they > * should become splitting first, unmapped, merged, > * and mapped back in on-demand. > */ > VM_BUG_ON(pmd_pfn(old_pmd) != pmd_pfn(*new_pmd)); > > pmd_clear(pmd); > for (cnt = 0; cnt < 512; cnt++) > kvm_tlb_flush_vmid_ipa(kvm, addr + cnt*PAGE_SIZE); > > Then the problem no longer reproduce. This makes very little sense. We shouldn't be able to enter this path for anything else but a permission update, otherwise the VM_BUG_ON should fire. Can you either turn this VM_BUG_ON into a simple BUG_ON, or enable CONFIG_DEBUG_VM please? If what you're describing is indeed correct (and I have no reason to doubt you), it should fire. Thanks, M. -- Jazz is not dead. It just smells funny... _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
