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Date:   Fri, 9 Dec 2022 09:08:48 +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 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?

-- 
Michal Hocko
SUSE Labs