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

[Mike Kravetz]

On 10/14/20 11:18 AM, David Hildenbrand wrote:
> On 14.10.20 19:56, Mina Almasry wrote:
>> On Wed, Oct 14, 2020 at 9:15 AM David Hildenbrand <david@xxxxxxxxxx> 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).

FWIW - I ran libhugetlbfs tests which do a bunch of hole punching
with (and without) hugetlb controller enabled and did not see this issue.

May need to reproduce via QEMU as below.
-- 
Mike Kravetz

> 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
>