Messages in this thread | | | Subject | Re: [RFC] Question about TLB flush while set Stage-2 huge pages | From | Suzuki K Poulose <> | Date | Fri, 15 Mar 2019 14:56:45 +0000 |
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Hi Zhengui,
On 15/03/2019 08:21, Zheng Xiang wrote: > Hi Suzuki, > > I have tested this patch, VM doesn't hang and we get expected WARNING log:
Thanks for the quick testing !
> However, we also get the following unexpected log: > > [ 908.329900] BUG: Bad page state in process qemu-kvm pfn:a2fb41cf > [ 908.339415] page:ffff7e28bed073c0 count:-4 mapcount:0 mapping:0000000000000000 index:0x0 > [ 908.339416] flags: 0x4ffffe0000000000() > [ 908.339418] raw: 4ffffe0000000000 dead000000000100 dead000000000200 0000000000000000 > [ 908.339419] raw: 0000000000000000 0000000000000000 fffffffcffffffff 0000000000000000 > [ 908.339420] page dumped because: nonzero _refcount > [ 908.339437] CPU: 32 PID: 72599 Comm: qemu-kvm Kdump: loaded Tainted: G B W 5.0.0+ #1 > [ 908.339438] Call trace: > [ 908.339439] dump_backtrace+0x0/0x188 > [ 908.339441] show_stack+0x24/0x30 > [ 908.339442] dump_stack+0xa8/0xcc > [ 908.339443] bad_page+0xf0/0x150 > [ 908.339445] free_pages_check_bad+0x84/0xa0 > [ 908.339446] free_pcppages_bulk+0x4b8/0x750 > [ 908.339448] free_unref_page_commit+0x13c/0x198 > [ 908.339449] free_unref_page+0x84/0xa0 > [ 908.339451] __free_pages+0x58/0x68 > [ 908.339452] zap_huge_pmd+0x290/0x2d8 > [ 908.339454] unmap_page_range+0x2b4/0x470 > [ 908.339455] unmap_single_vma+0x94/0xe8 > [ 908.339457] unmap_vmas+0x8c/0x108 > [ 908.339458] exit_mmap+0xd4/0x178 > [ 908.339459] mmput+0x74/0x180 > [ 908.339460] do_exit+0x2b4/0x5b0 > [ 908.339462] do_group_exit+0x3c/0xe0 > [ 908.339463] __arm64_sys_exit_group+0x24/0x28 > [ 908.339465] el0_svc_common+0xa0/0x180 > [ 908.339466] el0_svc_handler+0x38/0x78 > [ 908.339467] el0_svc+0x8/0xc
Thats bad, we seem to be making upto 4 unbalanced put_page().
>>> --- >>> virt/kvm/arm/mmu.c | 51 +++++++++++++++++++++++++++++++++++---------------- >>> 1 file changed, 35 insertions(+), 16 deletions(-) >>> >>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c >>> index 66e0fbb5..04b0f9b 100644 >>> --- a/virt/kvm/arm/mmu.c >>> +++ b/virt/kvm/arm/mmu.c >>> @@ -1076,24 +1076,38 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache >>> * Skip updating the page table if the entry is >>> * unchanged. >>> */ >>> - if (pmd_val(old_pmd) == pmd_val(*new_pmd)) >>> + if (pmd_val(old_pmd) == pmd_val(*new_pmd)) { >>> return 0; >>> - >>> + } else if (WARN_ON_ONCE(!pmd_thp_or_huge(old_pmd))) { >>> /* >>> - * 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. >>> + * If we have PTE level mapping for this block, >>> + * we must unmap it to avoid inconsistent TLB >>> + * state. We could end up in this situation if >>> + * the memory slot was marked for dirty logging >>> + * and was reverted, leaving PTE level mappings >>> + * for the pages accessed during the period. >>> + * Normal THP split/merge follows mmu_notifier >>> + * callbacks and do get handled accordingly. >>> */ >>> - VM_BUG_ON(pmd_pfn(old_pmd) != pmd_pfn(*new_pmd)); >>> + unmap_stage2_range(kvm, (addr & S2_PMD_MASK), S2_PMD_SIZE); > > It seems that kvm decreases the _refcount of the page twice in transparent_hugepage_adjust() > and unmap_stage2_range().
But I thought we should be doing that on the head_page already, as this is THP. I will take a look and get back to you on this. Btw, is it possible for you to turn on CONFIG_DEBUG_VM and re-run with the above patch ?
Kind regards Suzuki
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