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Date:   Fri, 9 Dec 2022 22:16:08 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Wei Xu <weixugc@google.com>
Cc:     Mina Almasry <almasrymina@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Huang Ying <ying.huang@intel.com>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Yosry Ahmed <yosryahmed@google.com>, fvdl@google.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] [mm-unstable] mm: Fix memcg reclaim on memory tiered
 systems
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On Fri 09-12-22 08:41:47, Wei Xu wrote:
> On Fri, Dec 9, 2022 at 12:08 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Thu 08-12-22 16:59:36, Wei Xu wrote:
> > [...]
> > > > What I really mean is to add demotion nodes to the nodemask along with
> > > > the set of nodes you want to reclaim from. To me that sounds like a
> > > > more natural interface allowing for all sorts of usecases:
> > > > - free up demotion targets (only specify demotion nodes in the mask)
> > > > - control where to demote (e.g. select specific demotion target(s))
> > > > - do not demote at all (skip demotion nodes from the node mask)
> > >
> > > For clarification, do you mean to add another argument (e.g.
> > > demotion_nodes) in addition to the "nodes" argument?
> >
> > No, nodes=mask argument should control the domain where the memory
> > reclaim should happen. That includes both aging and the reclaim. If the
> > mask doesn't contain any lower tier node then no demotion will happen.
> > If only a subset of lower tiers are specified then only those could be
> > used for the demotion process. Or put it otherwise, the nodemask is not
> > only used to filter out zonelists during reclaim it also restricts
> > migration targets.
> >
> > Is this more clear now?
> 
> In that case, how can we request demotion only from toptier nodes
> (without counting any reclaimed bytes from other nodes),  which is our
> memory tiering use case?

I am not sure I follow. Could you be more specific please?

> Besides, when both toptier and demotion nodes are specified, the
> demoted pages should only be counted as aging and not be counted
> towards the requested bytes of try_to_free_mem_cgroup_pages(), which
> is what this patch tries to address.

This should be addressed by
http://lkml.kernel.org/r/Y5B1K5zAE0PkjFZx@dhcp22.suse.cz, no?
-- 
Michal Hocko
SUSE Labs