linux-kernel - Re: [PATCH v2 1/2] hugetlb: memcg: account hugetlb-backed memory in memory controller

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Message-ID: <20231002152555.GA5054@cmpxchg.org>
Date:   Mon, 2 Oct 2023 11:25:55 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     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, mike.kravetz@...cle.com, yosryahmed@...gle.com,
        linux-mm@...ck.org, kernel-team@...a.com,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH v2 1/2] hugetlb: memcg: account hugetlb-backed memory in
 memory controller

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 from the
> > >   pool) which means that the page allocation might disrupt workloads
> > >   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 = dequeue();
	if (!folio) {
		folio = alloc_buddy();
		if (!folio)
			goto uncharge;
	}

	mem_cgroup_hugetlb_commit_charge();
}