Re: [PATCH v12 1/9] hugetlb_cgroup: Add hugetlb_cgroup reservation counter

[Mike Kravetz <mike.kravetz-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>]

On 2/18/20 10:35 AM, Mina Almasry wrote:
> On Tue, Feb 18, 2020 at 6:21 AM Qian Cai <cai-J5quhbR+WMc@public.gmane.org> wrote:
>>
>> On Tue, 2020-02-11 at 15:19 -0800, Andrew Morton wrote:
>>> On Tue, 11 Feb 2020 13:31:20 -0800 Mina Almasry <almasrymina-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
>>>
>> [ 7933.806377][T14355] ------------[ cut here ]------------
>> [ 7933.806541][T14355] kernel BUG at mm/hugetlb.c:490!
>> VM_BUG_ON(t - f <= 1);
>> [ 7933.806562][T14355] Oops: Exception in kernel mode, sig: 5 [#1]
<snip>
> Hi Qian,
> 
> Yes this VM_BUG_ON was added by a patch in the series ("hugetlb:
> disable region_add file_region coalescing") so it's definitely related
> to the series. I'm taking a look at why this VM_BUG_ON fires. Can you
> confirm you reproduce this by running hugemmap06 from the ltp on a
> powerpc machine? Can I maybe have your config?
> 
> Thanks!

Hi Mina,

Looking at the region_chg code again, we do a

	resv->adds_in_progress += *out_regions_needed;

and then potentially drop the lock to allocate the needed entries.  Could
anopther thread (only adding reservation for a single page) then come in
and notice that there are not enough entries in the cache and hit the
VM_BUG_ON()?

-- 
Mike Kravetz