Re: [RFC PATCH] hugetlbfs: Add hugetlb_cgroup reservation limits

[Mike Kravetz]

On 8/9/19 11:05 AM, Mina Almasry wrote:
> On Fri, Aug 9, 2019 at 4:27 AM Michal Koutnà <mkoutny@xxxxxxxx> wrote:
>>> Alternatives considered:
>>> [...]
>> (I did not try that but) have you considered:
>> 3) MAP_POPULATE while you're making the reservation,
> 
> I have tried this, and the behaviour is not great. Basically if
> userspace mmaps more memory than its cgroup limit allows with
> MAP_POPULATE, the kernel will reserve the total amount requested by
> the userspace, it will fault in up to the cgroup limit, and then it
> will SIGBUS the task when it tries to access the rest of its
> 'reserved' memory.
> 
> So for example:
> - if /proc/sys/vm/nr_hugepages == 10, and
> - your cgroup limit is 5 pages, and
> - you mmap(MAP_POPULATE) 7 pages.
> 
> Then the kernel will reserve 7 pages, and will fault in 5 of those 7
> pages, and will SIGBUS you when you try to access the remaining 2
> pages. So the problem persists. Folks would still like to know they
> are crossing the limits on mmap time.

If you got the failure at mmap time in the MAP_POPULATE case would this
be useful?

Just thinking that would be a relatively simple change.
-- 
Mike Kravetz