Re: [PATCH 0/2] hugetlb memcg accounting

Johannes Weiner
2023-10-02T14:42:56Z

On Sun, Oct 01, 2023 at 04:27:30PM -0700, Mike Kravetz wrote:
> On 09/27/23 14:47, Johannes Weiner wrote:
> > On Wed, Sep 27, 2023 at 01:21:20PM +0200, Michal Hocko wrote:
> > > On Tue 26-09-23 12:49:47, Nhat Pham wrote:
> >
> > So that if you use 80% hugetlb, the other memory is forced to stay in
> > the remaining 20%, or it OOMs; and that if you don't use hugetlb, the
> > group is still allowed to use the full 100% of its host memory
> > allowance, without requiring some outside agent continuously
> > monitoring and adjusting the container limits.
>
> Jumping in late here as I was traveling last week. In addition, I want
> to state my limited cgroup knowledge up front.
>
> I was thinking of your scenario above a little differently. Suppose a
> group is up and running at almost 100% memory usage. However, the majority
> of that memory is reclaimable. Now, someone wants to allocate a 2M hugetlb
> page. There is not 2MB free, but we could easily reclaim 2MB to make room
> for the hugetlb page. I may be missing something, but I do not see how that
> is going to happen. It seems like we would really want that behavior.

But that is actually what it does, no?

alloc_hugetlb_folio
mem_cgroup_hugetlb_charge_folio
charge_memcg
try_charge
!page_counter_try_charge ?
!try_to_free_mem_cgroup_pages ?
mem_cgroup_oom

So it does reclaim when the hugetlb hits the cgroup limit. And if that
fails to make room, it OOMs the cgroup.

Or maybe I'm missing something?