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Date:   Wed, 26 Oct 2022 13:02:32 +0100
From:   Mel Gorman <mgorman@suse.de>
To:     David Hildenbrand <david@redhat.com>
Cc:     Doug Berger <opendmb@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Mike Rapoport <rppt@kernel.org>, Borislav Petkov <bp@suse.de>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@suse.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Florian Fainelli <f.fainelli@gmail.com>,
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        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH v3 0/9] mm: introduce Designated Movable Blocks
Message-ID: <20221026120232.bbhfwjm32qq4mh57@suse.de>
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 <b8a4cd98-8236-d6e4-ee36-550ae1c107ff@redhat.com>
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On Wed, Oct 26, 2022 at 01:11:40PM +0200, David Hildenbrand wrote:
> > In the appliance case, it doesn't matter if the intent is that "all
> > application data should use high bandwidth memory where possible and
> > the application phase behaviour is predictable" and that may very well
> > work fine for the users of the Broadcom platforms with multiple memory
> > controllers. It does not work at all for the general where access must
> > be restricted to a subset of tasks in a general system that can only be
> > controlled with memory policies.
> > 
> > The high bandwidth memory should be representated as a NUMA node, optionally
> > to create that node as ZONE_MOVABLE and relying on the zonelists to select
> > the movable zone as the first preference.
> 
> ... that boils down to my remark to tiered memory and eventually using
> devdax to expose this memory to the system and letting the admin decide to
> online it to ZONE_MOVABLE. Of course, that's just one way of doing it.
> 

I don't see this approach being inherently bad as such, particularly in
the appliance space where it is known in advance what exactly is running
and what the requirements are. It's not automagical but it's not worse
than specifying something like movablecore=100M@2G,100M@3G,1G@1024G. In
either case, knowledge of the address ranges needing special treatment is
required with the difference being that access to the special memory can
be restricted by policies in the general case.

-- 
Mel Gorman
SUSE Labs