linux-kernel - Re: [PATCH 0/2] hugetlb memcg accounting

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Date:   Mon, 2 Oct 2023 10:42:50 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Michal Hocko <mhocko@...e.com>, Nhat Pham <nphamcs@...il.com>,
        akpm@...ux-foundation.org, riel@...riel.com,
        roman.gushchin@...ux.dev, shakeelb@...gle.com,
        muchun.song@...ux.dev, tj@...nel.org, lizefan.x@...edance.com,
        shuah@...nel.org, yosryahmed@...gle.com, linux-mm@...ck.org,
        kernel-team@...a.com, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org
Subject: Re: [PATCH 0/2] hugetlb memcg accounting

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?