[SRU][B][F][PATCH 0/1] KVM: Fix zero_page reference counter overflow when using KSM on KVM compute host

Matthew Ruffell matthew.ruffell at canonical.com
Sun Aug 16 23:51:53 UTC 2020

BugLink: https://bugs.launchpad.net/bugs/1837810


We are seeing a problem on OpenStack compute nodes, and KVM hosts, where a kernel
oops is generated, and all running KVM machines are placed into the pause state.

This is caused by the kernel's reserved zero_page reference counter overflowing
from a positive number to a negative number, and hitting a 
(WARN_ON_ONCE(page_ref_count(page) <= 0)) condition in try_get_page().

This only happens if the machine has Kernel Samepage Mapping (KSM) enabled,
with "use_zero_pages" turned on. Each time a new VM starts and the kernel does
a KSM merge run during a EPT violation, the reference counter for the zero_page
is incremented in try_async_pf() and never decremented. Eventually, the reference
counter will overflow, causing the KVM subsystem to fail.

error : qemuMonitorJSONCheckError:392 : internal error: unable to execute QEMU command 'cont': Resetting the Virtual Machine is required

QEMU Logs:
error: kvm run failed Bad address
EAX=000afe00 EBX=0000000b ECX=00000080 EDX=00000cfe
ESI=0003fe00 EDI=000afe00 EBP=00000007 ESP=00006d74
EIP=000ee344 EFL=00010002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
ES =0010 00000000 ffffffff 00c09300 DPL=0 DS   [-WA]
CS =0008 00000000 ffffffff 00c09b00 DPL=0 CS32 [-RA]
SS =0010 00000000 ffffffff 00c09300 DPL=0 DS   [-WA]
DS =0010 00000000 ffffffff 00c09300 DPL=0 DS   [-WA]
FS =0010 00000000 ffffffff 00c09300 DPL=0 DS   [-WA]
GS =0010 00000000 ffffffff 00c09300 DPL=0 DS   [-WA]
LDT=0000 00000000 0000ffff 00008200 DPL=0 LDT
TR =0000 00000000 0000ffff 00008b00 DPL=0 TSS32-busy
GDT=     000f7040 00000037
IDT=     000f707e 00000000
CR0=00000011 CR2=00000000 CR3=00000000 CR4=00000000
DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000 DR3=0000000000000000 
DR6=00000000ffff0ff0 DR7=0000000000000400
Code=c3 57 56 b8 00 fe 0a 00 be 00 fe 03 00 b9 80 00 00 00 89 c7 <f3> a5 a1 00 80 03 00 8b 15 04 80 03 00 a3 00 80 0a 00 89 15 04 80 0a 00 b8 ae e2 00 00 31

Kernel Oops:

[  167.695986] WARNING: CPU: 1 PID: 3016 at /build/linux-hwe-FEhT7y/linux-hwe-4.15.0/include/linux/mm.h:852 follow_page_pte+0x6f4/0x710
[  167.696023] CPU: 1 PID: 3016 Comm: CPU 0/KVM Tainted: G           OE    4.15.0-106-generic #107~16.04.1-Ubuntu
[  167.696023] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1 04/01/2014
[  167.696025] RIP: 0010:follow_page_pte+0x6f4/0x710
[  167.696026] RSP: 0018:ffffa81802023908 EFLAGS: 00010286
[  167.696027] RAX: ffffed8786e33a80 RBX: ffffed878c6d21b0 RCX: 0000000080000000
[  167.696027] RDX: 0000000000000000 RSI: 00003ffffffff000 RDI: 80000001b8cea225
[  167.696028] RBP: ffffa81802023970 R08: 80000001b8cea225 R09: ffff90c4d55fa340
[  167.696028] R10: 0000000000000000 R11: 0000000000000000 R12: ffffed8786e33a80
[  167.696029] R13: 0000000000000326 R14: ffff90c4db94fc50 R15: ffff90c4d55fa340
[  167.696030] FS:  00007f6a7798c700(0000) GS:ffff90c4edc80000(0000) knlGS:0000000000000000
[  167.696030] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  167.696031] CR2: 0000000000000000 CR3: 0000000315580002 CR4: 0000000000162ee0
[  167.696033] Call Trace:
[  167.696047]  follow_pmd_mask+0x273/0x630
[  167.696049]  follow_page_mask+0x178/0x230
[  167.696051]  __get_user_pages+0xb8/0x740
[  167.696052]  get_user_pages+0x42/0x50
[  167.696068]  __gfn_to_pfn_memslot+0x18b/0x3b0 [kvm]
[  167.696079]  ? mmu_set_spte+0x1dd/0x3a0 [kvm]
[  167.696090]  try_async_pf+0x66/0x220 [kvm]
[  167.696101]  tdp_page_fault+0x14b/0x2b0 [kvm]
[  167.696104]  ? vmexit_fill_RSB+0x10/0x40 [kvm_intel]
[  167.696114]  kvm_mmu_page_fault+0x62/0x180 [kvm]
[  167.696117]  handle_ept_violation+0xbc/0x160 [kvm_intel]
[  167.696119]  vmx_handle_exit+0xa5/0x580 [kvm_intel]
[  167.696129]  vcpu_enter_guest+0x414/0x1260 [kvm]
[  167.696138]  ? kvm_arch_vcpu_load+0x4d/0x280 [kvm]
[  167.696148]  kvm_arch_vcpu_ioctl_run+0xd9/0x3d0 [kvm]
[  167.696157]  ? kvm_arch_vcpu_ioctl_run+0xd9/0x3d0 [kvm]
[  167.696165]  kvm_vcpu_ioctl+0x33a/0x610 [kvm]
[  167.696166]  ? do_futex+0x129/0x590
[  167.696171]  ? __switch_to+0x34c/0x4e0
[  167.696174]  ? __switch_to_asm+0x35/0x70
[  167.696176]  do_vfs_ioctl+0xa4/0x600
[  167.696177]  SyS_ioctl+0x79/0x90
[  167.696180]  ? exit_to_usermode_loop+0xa5/0xd0
[  167.696181]  do_syscall_64+0x73/0x130
[  167.696182]  entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[  167.696184] RIP: 0033:0x7f6a80482007
[  167.696184] RSP: 002b:00007f6a7798b8b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[  167.696185] RAX: ffffffffffffffda RBX: 000000000000ae80 RCX: 00007f6a80482007
[  167.696185] RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000016
[  167.696186] RBP: 000055fe135f3240 R08: 000055fe118be530 R09: 0000000000000001
[  167.696186] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
[  167.696187] R13: 00007f6a85852000 R14: 0000000000000000 R15: 000055fe135f3240
[  167.696188] Code: 4d 63 e6 e9 f2 fc ff ff 4c 89 45 d0 48 8b 47 10 e8 22 f0 9e 00 4c 8b 45 d0 e9 89 fc ff ff 4c 89 e7 e8 81 3f fd ff e9 aa fc ff ff <0f> 0b 49 c7 c4 f4 ff ff ff e9 c1 fc ff ff 0f 1f 40 00 66 2e 0f 
[  167.696200] ---[ end trace 7573f6868ea8f069 ]---


This was fixed in 5.6-rc1 with the following commit:

commit 7df003c85218b5f5b10a7f6418208f31e813f38f
Author: Zhuang Yanying <ann.zhuangyanying at huawei.com>
Date:   Sat Oct 12 11:37:31 2019 +0800
Subject:  KVM: fix overflow of zero page refcount with ksm running
Link: https://github.com/torvalds/linux/commit/7df003c85218b5f5b10a7f6418208f31e813f38f 

The fix adds a check to see if the Page Frame Number (pfn) is linked to the zero
page, and if it is, treats it as reserved. This has the effect that put_page()
is no longer called on the zero_page, and reference counting is no longer needed.

This is a clean cherry pick to Bionic and Focal kernels.


Create a new KVM host, and make sure it has plenty of ram. 16gb should be okay.

Install KVM packages:

$ sudo apt install -y qemu-kvm libvirt-bin qemu-utils genisoimage virtinst

Enable Kernel Samepage Mapping, and use_zero_pages:

$ echo 10000 | sudo tee /sys/kernel/mm/ksm/pages_to_scan
$ echo 1 | sudo tee /sys/kernel/mm/ksm/run
$ echo 1 | sudo tee /sys/kernel/mm/ksm/use_zero_pages

I wrote a script which creates and destroys xenial KVM VMs in a infinite loop:

Save the script to disk, and execute it:

$ chmod +x ksm_refcnt_overflow.sh
$ ./ksm_refcnt_overflow.sh

Each time a VM is created and destroyed the reference counter will increase.

I wrote a kernel module which exposes a /proc interface, which we can use to
look at the value of the zero_page reference counter. It works by taking the
memory allocated for the zero page: empty_zero_page, which is defined in
arch/x86/include/asm/pgtable.h, running virt_to_page() to get the page struct,
which we can then dereference to get _refcount;


Save the module to disk, create its Makefile from the included documentation,
and build it:

$ make
$ sudo insmod zero_page_refcount.ko 

>From there, we can examine the reference counter with:

$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x687 or 1671
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x846 or 2118
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x9f8 or 2552
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0xcb2 or 3250 

We see it steadily increase. Instead of waiting months for it to overflow, I
implemented a /proc entry to set it to near overflow. You can use it with:

$ cat /proc/zero_page_refcount_set
Zero Page Refcount set to 0x1FFFFFFFFF000 

After that, wait a few seconds and the reference counter will overflow:

$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x7fffff16 or 2147483414
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x80000000 or -2147483648 

All VMs will become paused:

$ virsh list
Id Name State
1 instance-0 paused
2 instance-1 paused 

QEMU will error out, and the kernel will oops with the messages in the impact

I built a test kernel, which is available here:


If you install the test kernel and try reproduce, you will notice the reference
counter is never incremented past 1:

$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x1 or 1
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x1 or 1
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x1 or 1 

This resolves the problem.

[Regression Potential]

While the change itself seems simple, it changes how the kernel treats the
zero_page. The zero_page is important, since it is just a page full of 0's.
Each time memory is allocated which is all 0s, the kernel sets it to use the
zero_page to save memory. When an application writes to the buffer, a EPT
violation happens, and the kernel does a COW to new pages to hold the data.

The change is limited to how the KVM subsystem handles the zero_page.
This will not break the entire kernel if a regression occurs, only KVM.

If a regression were to occur, users could turn off KSM and disable KSM
use_zero_pages until a fix is ready, as this particular use of zero_pages is
limited to KSM.

The fix landed in upstream 5.6, and has not been backported to stable kernels.

I have read a bit of the paging code, especially around where the zero_page is
used, and where its reference counters were being incorrectly incremented.
I think the fix is correct, and I believe it won't cause any regressions.

Zhuang Yanying (1):
  KVM: fix overflow of zero page refcount with ksm running

 virt/kvm/kvm_main.c | 1 +
 1 file changed, 1 insertion(+)


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