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 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%).

-- 
Thanks,

David / dhildenb