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]

On 15.10.20 10:57, David Hildenbrand wrote:
> On 15.10.20 09:56, David Hildenbrand wrote:
>> On 14.10.20 20:31, Mike Kravetz wrote:
>>> 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.
>>
>> Not sure if relevant, but QEMU should be using
>> memfd_create(MFD_HUGETLB|MFD_HUGE_2MB) to obtain a hugetlbfs file.
>>
>> Also, QEMU fallocate(FALLOC_FL_PUNCH_HOLE)'s a significant of memory of
>> the md (e.g., > 90%).
>>
> 
> I just tried to reproduce by doing random accesses + random fallocate(FALLOC_FL_PUNCH_HOLE) within a file - without success.
> 
> So could be
> 1. KVM is involved messing this up
> 2. Multi-threading is involved
> 

Able to reproduce with TCG under QEMU, so not a KVM issue.

-- 
Thanks,

David / dhildenb