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

[Mina Almasry]

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?

>
> Just tried without the hugetlb controller, seems to work just fine.
>
> I'd like to note that
> - The controller was not activated
> - I had to compile the hugetlb controller out to make it work.
>
> --
> Thanks,
>
> David / dhildenb
>