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

[Mina Almasry]

On Fri, Aug 9, 2019 at 1:39 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> 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.

Not quite, unfortunately. A subset of the folks that want to use
hugetlb memory, don't want to use MAP_POPULATE (IIRC, something about
mmaping a huge amount of hugetlb memory at their jobs' startup, and
doing that with MAP_POPULATE adds so much to their startup time that
it is prohibitively expensive - but that's just what I vaguely recall
offhand. I can get you the details if you're interested).

> --
> Mike Kravetz