Re: cgroup and FALLOC_FL_PUNCH_HOLE: WARNING: CPU: 13 PID: 2438 at mm/page_counter.c:57 page_counter_uncharge+0x4b/0x5

[David Hildenbrand <david@redhat.com>]

On 14.10.20 19:56, Mina Almasry wrote:
> On Wed, Oct 14, 2020 at 9:15 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 14.10.20 17:22, David Hildenbrand wrote:
>>> Hi everybody,
>>>
>>> Michal Privoznik played with "free page reporting" in QEMU/virtio-balloon
>>> with hugetlbfs and reported that this results in [1]
>>>
>>> 1. WARNING: CPU: 13 PID: 2438 at mm/page_counter.c:57 page_counter_uncharge+0x4b/0x5
>>>
>>> 2. Any hugetlbfs allocations failing. (I assume because some accounting is wrong)
>>>
>>>
>>> QEMU with free page hinting uses fallocate(FALLOC_FL_PUNCH_HOLE)
>>> to discard pages that are reported as free by a VM. The reporting
>>> granularity is in pageblock granularity. So when the guest reports
>>> 2M chunks, we fallocate(FALLOC_FL_PUNCH_HOLE) one huge page in QEMU.
>>>
>>> I was also able to reproduce (also with virtio-mem, which similarly
>>> uses fallocate(FALLOC_FL_PUNCH_HOLE)) on latest v5.9
>>> (and on v5.7.X from F32).
>>>
>>> Looks like something with fallocate(FALLOC_FL_PUNCH_HOLE) accounting
>>> is broken with cgroups. I did *not* try without cgroups yet.
>>>
>>> Any ideas?
> 
> Hi David,
> 
> I may be able to dig in and take a look. How do I reproduce this
> though? I just fallocate(FALLOC_FL_PUNCH_HOLE) one 2MB page in a
> hugetlb region?
> 

Hi Mina,

thanks for having a look. I started poking around myself but,
being new to cgroup code, I even failed to understand why that code gets
triggered though the hugetlb controller isn't even enabled.

I assume you at least have to make sure that there is
a page populated (MMAP_POPULATE, or read/write it). But I am not
sure yet if a single fallocate(FALLOC_FL_PUNCH_HOLE) is
sufficient, or if it will require a sequence of
populate+discard(punch) (or multi-threading).

What definitely makes it trigger is via QEMU

qemu-system-x86_64 \
-machine pc-i440fx-4.0,accel=kvm,usb=off,dump-guest-core=off,memory-backend=pc.ram \
-cpu host,migratable=on \
-m 4096 \
-object memory-backend-memfd,id=pc.ram,hugetlb=yes,hugetlbsize=2097152,size=4294967296 \
-overcommit mem-lock=off \
-smp 4,sockets=1,dies=1,cores=2,threads=2 \
-nodefaults \
-nographic \
-device virtio-scsi-pci,id=scsi0,bus=pci.0,addr=0x4 \
-device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x5 \
-blockdev '{"driver":"file","filename":"../Fedora-Cloud-Base-32-1.6.x86_64.qcow2","node-name":"libvirt-1-storage","auto-read-only":true,"discard":"unmap"}' \
-blockdev '{"node-name":"libvirt-1-format","read-only":false,"discard":"unmap","driver":"qcow2","file":"libvirt-1-storage","backing":null}' \
-device scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=0,device_id=drive-scsi0-0-0-0,drive=libvirt-1-format,id=scsi0-0-0-0,bootindex=1 \
-chardev stdio,nosignal,id=serial \
-device isa-serial,chardev=serial \
-device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x7,free-page-reporting=on


However, you need a recent QEMU (>= v5.1 IIRC) and a recent kernel
(>= v5.7) inside your guest image.

Fedora rawhide qcow2 should do: https://dl.fedoraproject.org/pub/fedora/linux/development/rawhide/Cloud/x86_64/images/Fedora-Cloud-Base-Rawhide-20201004.n.1.x86_64.qcow2


-- 
Thanks,

David / dhildenb