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

From: Zenghui Yu
Date: Sun Mar 17 2019 - 10:02:40 EST

Hi Suzuki,

On 2019/3/15 22:56, Suzuki K Poulose wrote:
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
[Â 908.339415] page:ffff7e28bed073c0 count:-4 mapcount:0 mapping:0000000000000000 index:0x0
[Â 908.339416] flags: 0x4ffffe0000000000()
[Â 908.339418] raw: 4ffffe0000000000 dead000000000100 dead000000000200 0000000000000000
[Â 908.339419] raw: 0000000000000000 0000000000000000 fffffffcffffffff 0000000000000000
[Â 908.339420] page dumped because: nonzero _refcount
[Â 908.339437] CPU: 32 PID: 72599 Comm: qemu-kvm Kdump: loaded Tainted: GÂÂÂ BÂ WÂÂÂÂÂÂÂ 5.0.0+ #1
[Â 908.339438] Call trace:
[Â 908.339439]Â dump_backtrace+0x0/0x188
[Â 908.339441]Â show_stack+0x24/0x30
[Â 908.339442]Â dump_stack+0xa8/0xcc
[Â 908.339443]Â bad_page+0xf0/0x150
[Â 908.339445]Â free_pages_check_bad+0x84/0xa0
[Â 908.339446]Â free_pcppages_bulk+0x4b8/0x750
[Â 908.339448]Â free_unref_page_commit+0x13c/0x198
[Â 908.339449]Â free_unref_page+0x84/0xa0
[Â 908.339451]Â __free_pages+0x58/0x68
[Â 908.339452]Â zap_huge_pmd+0x290/0x2d8
[Â 908.339454]Â unmap_page_range+0x2b4/0x470
[Â 908.339455]Â unmap_single_vma+0x94/0xe8
[Â 908.339457]Â unmap_vmas+0x8c/0x108
[Â 908.339458]Â exit_mmap+0xd4/0x178
[Â 908.339459]Â mmput+0x74/0x180
[Â 908.339460]Â do_exit+0x2b4/0x5b0
[Â 908.339462]Â do_group_exit+0x3c/0xe0
[Â 908.339463]Â __arm64_sys_exit_group+0x24/0x28
[Â 908.339465]Â el0_svc_common+0xa0/0x180
[Â 908.339466]Â el0_svc_handler+0x38/0x78
[Â 908.339467]Â el0_svc+0x8/0xc

Thats bad, we seem to be making upto 4 unbalanced put_page().

---
ÂÂ virt/kvm/arm/mmu.c | 51 +++++++++++++++++++++++++++++++++++----------------
ÂÂ 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
ÂÂÂÂÂÂÂÂÂÂÂ * Skip updating the page table if the entry is
ÂÂÂÂÂÂÂÂÂÂÂ * unchanged.
ÂÂÂÂÂÂÂÂÂÂÂ */
-ÂÂÂÂÂÂÂ if (pmd_val(old_pmd) == pmd_val(*new_pmd))
+ÂÂÂÂÂÂÂ if (pmd_val(old_pmd) == pmd_val(*new_pmd)) {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ return 0;
-
+ÂÂÂÂÂÂÂ } else if (WARN_ON_ONCE(!pmd_thp_or_huge(old_pmd))) {
ÂÂÂÂÂÂÂÂÂÂ /*
-ÂÂÂÂÂÂÂÂ * Mapping in huge pages should only happen through a
- * fault. If a page is merged into a transparent huge
-ÂÂÂÂÂÂÂÂ * page, the individual subpages of that huge page
-ÂÂÂÂÂÂÂÂ * should be unmapped through MMU notifiers before we
-ÂÂÂÂÂÂÂÂ * get here.
-ÂÂÂÂÂÂÂÂ *
-ÂÂÂÂÂÂÂÂ * Merging of CompoundPages is not supported; they
-ÂÂÂÂÂÂÂÂ * should become splitting first, unmapped, merged,
-ÂÂÂÂÂÂÂÂ * and mapped back in on-demand.
+ÂÂÂÂÂÂÂÂ * If we have PTE level mapping for this block,
+ÂÂÂÂÂÂÂÂ * we must unmap it to avoid inconsistent TLB
+ÂÂÂÂÂÂÂÂ * state. We could end up in this situation if
+ÂÂÂÂÂÂÂÂ * the memory slot was marked for dirty logging
+ÂÂÂÂÂÂÂÂ * and was reverted, leaving PTE level mappings
+ÂÂÂÂÂÂÂÂ * for the pages accessed during the period.
+ÂÂÂÂÂÂÂÂ * Normal THP split/merge follows mmu_notifier
+ÂÂÂÂÂÂÂÂ * callbacks and do get handled accordingly.
ÂÂÂÂÂÂÂÂÂÂÂ */
-ÂÂÂÂÂÂÂ VM_BUG_ON(pmd_pfn(old_pmd) != pmd_pfn(*new_pmd));
+ÂÂÂÂÂÂÂÂÂÂÂ 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 ?

And for detailed debugging info:

I've turned on CONFIG_DEBUG_VM and re-run with your patch -- Run a guest
with stage2 PUD hugepage, enable then disable the dirty logging, and
then shutdown this guest. The result is: Host hit a kernel BUG with the
below log (when shutdown-ing guest):


[ 486.997640] kernel BUG at ./include/linux/mm.h:547!
[ 487.005524] Internal error: Oops - BUG: 0 [#1] SMP
[ 487.013455] Modules linked in: ...
[ 487.104072] CPU: 14 PID: 60747 Comm: qemu-kvm Kdump: loaded Tainted: G W 5.0.0+ #2
[ 487.117150] ...
[ 487.135433] pstate: 40400009 (nZcv daif +PAN -UAO)
[ 487.144849] pc : unmap_stage2_puds+0x480/0x6e0
[ 487.153756] lr : unmap_stage2_puds+0x480/0x6e0
[ 487.162507] sp : ffff00002c72bb10
[ 487.179630] x27: 0000000041a00000 x26: ffff8027bbb56060
[ 487.183465] openvswitch: netlink: Tunnel attr 5 has unexpected len 1 expected 0
[ 487.189184] x25: ffff802769cbe008 x24: ffff7e0000000000
[ 487.189185] x23: ffff802769cbe008 x22: ffff00004b0af000
[ 487.189186] x21: ffff80279da06060 x20: 00400027332007fd
[ 487.189188] x19: 0000000080000000 x18: 0000000000000010
[ 487.189189] x17: 0000000000000000 x16: 0000000000000000
[ 487.189190] x15: ffff00001182d708 x14: 3030303030303030
[ 487.189191] x13: 3030303030302066 x12: ffff000011857000
[ 487.189192] x11: 0000000000000000 x10: ffff000011a48000
[ 487.189193] x9 : 0000000000000000 x8 : 0000000000000003
[ 487.189194] x7 : 000000000000095b x6 : 0000000212557560
[ 487.189196] x3 : ffff802fc0b08260 x2 : b2513adc3568f800
[ 487.189197] x1 : 0000000000000000 x0 : 000000000000003e
[ 487.189200] Process qemu-kvm (pid: 60747, stack limit = 0x000000004342b298)
[ 487.189201] Call trace:
[ 487.189203] unmap_stage2_puds+0x480/0x6e0
[ 487.189205] unmap_stage2_range+0xa4/0x190
[ 487.189208] kvm_free_stage2_pgd+0x64/0x100
[ 487.363897] kvm_arch_flush_shadow_all+0x20/0x30
[ 487.372095] kvm_mmu_notifier_release+0x3c/0x80
[ 487.380092] __mmu_notifier_release+0x50/0x100
[ 487.387914] exit_mmap+0x170/0x178
[ 487.394567] mmput+0x70/0x180
[ 487.400653] do_exit+0x2b4/0x5c8
[ 487.406849] do_group_exit+0x3c/0xe0
--[ end trace 55c414a329c80b63 ]---
[ 487.454174] Kernel panic - not syncing: Fatal exception
[ 487.461967] SMP: stopping secondary CPUs
[ 487.468473] Kernel Offset: disabled
[ 487.474520] CPU features: 0x002,22208a38
[ 487.481008] Memory Limit: none
[ 487.489095] Starting crashdump kernel...
[ 487.495457] Bye!



Kind regards
Suzuki



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