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 11:31 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.
-- 
Mike Kravetz