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From:   Nhat Pham <nphamcs@gmail.com>
Date:   Mon, 2 Oct 2023 10:32:06 -0700
Message-ID: <CAKEwX=NLBs=Oi8VDUyawWvKKhwU08kbgq8RKrY8CqrcG9G2QWA@mail.gmail.com>
Subject: Re: [PATCH v2 1/2] hugetlb: memcg: account hugetlb-backed memory in
 memory controller
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Michal Hocko <mhocko@suse.com>, akpm@linux-foundation.org,
        riel@surriel.com, roman.gushchin@linux.dev, shakeelb@google.com,
        muchun.song@linux.dev, tj@kernel.org, lizefan.x@bytedance.com,
        shuah@kernel.org, mike.kravetz@oracle.com, yosryahmed@google.com,
        linux-mm@kvack.org, kernel-team@meta.com,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org
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On Mon, Oct 2, 2023 at 8:25=E2=80=AFAM Johannes Weiner <hannes@cmpxchg.org>=
 wrote:
>
> On Mon, Oct 02, 2023 at 05:08:34PM +0200, Michal Hocko wrote:
> > On Mon 02-10-23 10:50:26, Johannes Weiner wrote:
> > > On Mon, Oct 02, 2023 at 03:43:19PM +0200, Michal Hocko wrote:
> > > > On Wed 27-09-23 17:57:22, Nhat Pham wrote:
> > [...]
> > > > - memcg limit reclaim doesn't assist hugetlb pages allocation when
> > > >   hugetlb overcommit is configured (i.e. pages are not consumed fro=
m the
> > > >   pool) which means that the page allocation might disrupt workload=
s
> > > >   from other memcgs.
> > > > - failure to charge a hugetlb page results in SIGBUS rather
> > > >   than memcg oom killer. That could be the case even if the
> > > >   hugetlb pool still has pages available and there is
> > > >   reclaimable memory in the memcg.
> > >
> > > Are these actually true? AFAICS, regardless of whether the page comes
> > > from the pool or the buddy allocator, the memcg code will go through
> > > the regular charge path, attempt reclaim, and OOM if that fails.
> >
> > OK, I should have been more explicit. Let me expand. Charges are
> > accounted only _after_ the actual allocation is done. So the actual
> > allocation is not constrained by the memcg context. It might reclaim
> > from the memcg at that time but the disruption could have already
> > happened. Not really any different from regular memory allocation
> > attempt but much more visible with GB pages and one could reasonably
> > expect that memcg should stop such a GB allocation if the local reclaim
> > would be hopeless to free up enough from its own consumption.
> >
> > Makes more sense?
>
> Yes, that makes sense.
>
> This should be fairly easy to address by having hugetlb do the split
> transaction that charge_memcg() does in one go, similar to what we do
> for the hugetlb controller as well. IOW,
>
> alloc_hugetlb_folio()
> {
>         if (mem_cgroup_hugetlb_try_charge())
>                 return ERR_PTR(-ENOMEM);
>
>         folio =3D dequeue();
>         if (!folio) {
>                 folio =3D alloc_buddy();
>                 if (!folio)
>                         goto uncharge;
>         }
>
>         mem_cgroup_hugetlb_commit_charge();
> }

Ah actually, I like this better.
If I do this I can circumvent all the redo_reserve bogus as well!