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 16.10.20 01:14, Mike Kravetz wrote:
> On 10/14/20 11:31 AM, Mike Kravetz wrote:
>> On 10/14/20 11:18 AM, David Hildenbrand wrote:
>>
>> FWIW - I ran libhugetlbfs tests which do a bunch of hole punching
>> with (and without) hugetlb controller enabled and did not see this issue.
>>
> 
> I took a closer look after running just the fallocate_stress test
> in libhugetlbfs.  Here are the cgroup counter values:
> 
> hugetlb.2MB.failcnt 0
> hugetlb.2MB.limit_in_bytes 9223372036854771712
> hugetlb.2MB.max_usage_in_bytes 209715200
> hugetlb.2MB.rsvd.failcnt 0
> hugetlb.2MB.rsvd.limit_in_bytes 9223372036854771712
> hugetlb.2MB.rsvd.max_usage_in_bytes 601882624
> hugetlb.2MB.rsvd.usage_in_bytes 392167424
> hugetlb.2MB.usage_in_bytes 0
> 
> We did not hit the WARN_ON_ONCE(), but the 'rsvd.usage_in_bytes' value
> is not correct in that it should be zero.   No huge page reservations
> remain after the test.
> 
> HugePages_Total:    1024
> HugePages_Free:     1024
> HugePages_Rsvd:        0
> HugePages_Surp:        0
> Hugepagesize:       2048 kB
> Hugetlb:         2097152 kB
> 
> To try and better understand the reservation cgroup controller, I addded
> a few printks to the code.  While running fallocate_stress with the
> printks, I can consistently hit the WARN_ON_ONCE() due to the counter
> going negative.  Here are the cgroup counter values after such a run:
> 
> hugetlb.2MB.failcnt 0
> hugetlb.2MB.limit_in_bytes 9223372036854771712
> hugetlb.2MB.max_usage_in_bytes 209715200
> hugetlb.2MB.rsvd.failcnt 3
> hugetlb.2MB.rsvd.limit_in_bytes 9223372036854771712
> hugetlb.2MB.rsvd.max_usage_in_bytes 251658240
> hugetlb.2MB.rsvd.usage_in_bytes 18446744073487253504
> hugetlb.2MB.usage_in_bytes 0
> 
> Again, no reserved pages after the test.
> 
> HugePages_Total:    1024
> HugePages_Free:     1024
> HugePages_Rsvd:        0
> HugePages_Surp:        0
> Hugepagesize:       2048 kB
> Hugetlb:         2097152 kB
> 
> I have some basic hugetlb hole punch functionality tests.  Running
> these on the kernel with added printk's does not cause any issues.
> In order to reproduce, I need to run fallocate_stress test which
> will cause hole punch to race with page fault.  Best guess at this
> time is that some of the error/race detection reservation back out
> code is not properly dealing with cgroup accounting.
> 
> I'll take a look at this as well.
> 

I'm bisecting the warning right now. Looks like it was introduced in v5.7.

-- 
Thanks,

David / dhildenb