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Message-Id: <20240313085144.13b37a79c688b6126af0bd07@linux-foundation.org>
Date: Wed, 13 Mar 2024 08:51:44 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Donet Tom <donettom@...ux.ibm.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Aneesh Kumar <aneesh.kumar@...nel.org>,
Michal Hocko <mhocko@...nel.org>, Dave Hansen
<dave.hansen@...ux.intel.com>, Mel Gorman <mgorman@...e.de>, Feng Tang
<feng.tang@...el.com>, Andrea Arcangeli <aarcange@...hat.com>, Peter
Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Rik van
Riel <riel@...riel.com>, Johannes Weiner <hannes@...xchg.org>, Matthew
Wilcox <willy@...radead.org>, Vlastimil Babka <vbabka@...e.cz>, Dan
Williams <dan.j.williams@...el.com>, Hugh Dickins <hughd@...gle.com>,
Kefeng Wang <wangkefeng.wang@...wei.com>, Suren Baghdasaryan
<surenb@...gle.com>
Subject: Re: [PATCH v2 2/2] mm/numa_balancing:Allow migrate on protnone
reference with MPOL_PREFERRED_MANY policy
On Mon, 11 Mar 2024 09:37:36 +0800 "Huang, Ying" <ying.huang@...el.com> wrote:
> > @@ -2515,15 +2516,26 @@ int mpol_misplaced(struct folio *folio, struct vm_fault *vmf,
> > break;
> >
> > case MPOL_BIND:
> > - /* Optimize placement among multiple nodes via NUMA balancing */
> > + case MPOL_PREFERRED_MANY:
> > + /*
> > + * Even though MPOL_PREFERRED_MANY can allocate pages outside
> > + * policy nodemask we don't allow numa migration to nodes
> > + * outside policy nodemask for now. This is done so that if we
> > + * want demotion to slow memory to happen, before allocating
> > + * from some DRAM node say 'x', we will end up using a
> > + * MPOL_PREFERRED_MANY mask excluding node 'x'. In such scenario
> > + * we should not promote to node 'x' from slow memory node.
> > + */
>
> This is a little hard to digest for me. And, I don't think that we need
> to put this policy choice in code comments. It's better to put it in
> patch description. Where we can give more background, for example, to
> avoid cross-socket traffic, etc.
Oh. I like the comment. We could perhaps put additional detail in the
changelog, but using changelogs to understand the code is so darned
inconvenient.
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