Re: [RFC] Question about TLB flush while set Stage-2 huge pages

[Suzuki K Poulose <suzuki.poulose@arm.com>]

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