Re: [PATCH RFC] hugetlb_cgroup: fix unbalanced css_put for shared mappings

[Miaohe Lin <linmiaohe@huawei.com>]

On 2021/2/9 3:52, Mike Kravetz wrote:
> On 1/23/21 1:31 AM, Miaohe Lin wrote:
>> The current implementation of hugetlb_cgroup for shared mappings could have
>> different behavior. Consider the following two scenarios:
>>
>> 1.Assume initial css reference count of hugetlb_cgroup is 1:
>>   1.1 Call hugetlb_reserve_pages with from = 1, to = 2. So css reference
>> count is 2 associated with 1 file_region.
>>   1.2 Call hugetlb_reserve_pages with from = 2, to = 3. So css reference
>> count is 3 associated with 2 file_region.
>>   1.3 coalesce_file_region will coalesce these two file_regions into one.
>> So css reference count is 3 associated with 1 file_region now.
>>
>> 2.Assume initial css reference count of hugetlb_cgroup is 1 again:
>>   2.1 Call hugetlb_reserve_pages with from = 1, to = 3. So css reference
>> count is 2 associated with 1 file_region.
>>
>> Therefore, we might have one file_region while holding one or more css
>> reference counts. This inconsistency could lead to unbalanced css_put().
>> If we do css_put one by one (i.g. hole punch case), scenario 2 would put
>> one more css reference. If we do css_put all together (i.g. truncate case),
>> scenario 1 will leak one css reference.
> 
> Sorry for the delay in replying.  This is tricky code and I needed some quiet
> time to study it.
> 

That's fine. I was trying to catch more buggy case too.

> I agree that the issue described exists.  Can you describe what a user would
> see in the above imbalance scenarios?  What happens if we do one too many
> css_put calls?  What happens if we leak the reference and do not do the
> required number of css_puts?
> 

The imbalanced css_get/css_put would result in a non-zero reference when we try to
destroy the hugetlb cgroup. The hugetlb cgroup dir is removed __but__ associated
resource is not freed. This might result in OOM or can not create a new hugetlb cgroup
in a really busy workload finally.

> The code changes look correct.
> 
> I just wish this code was not so complicated.  I think the private mapping
> case could be simplified to only take a single css_ref per reserve map.

Could you explain this more?
It seems one reserve map already takes a single css_ref. And a hugepage outside
reservation would take a single css_ref too.

> However, for shared mappings we need to track each individual reservation
> which adds the complexity.  I can not think of a better way to do things.
> 

I can't figure out one too. And the fix might make the code more complex. :(

> Please update commit message with an explanation of what users might see
> because of this issue and resubmit as a patch.
> 

Will do. Thanks.

> Thanks,
> 

Many thanks for reply. :)