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Date: Wed, 13 Mar 2024 08:51:44 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: Donet Tom <donettom@linux.ibm.com>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org, Aneesh Kumar <aneesh.kumar@kernel.org>,
 Michal Hocko <mhocko@kernel.org>, Dave Hansen
 <dave.hansen@linux.intel.com>, Mel Gorman <mgorman@suse.de>, Feng Tang
 <feng.tang@intel.com>, Andrea Arcangeli <aarcange@redhat.com>, Peter
 Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>, Rik van
 Riel <riel@surriel.com>, Johannes Weiner <hannes@cmpxchg.org>, Matthew
 Wilcox <willy@infradead.org>, Vlastimil Babka <vbabka@suse.cz>, Dan
 Williams <dan.j.williams@intel.com>, Hugh Dickins <hughd@google.com>,
 Kefeng Wang <wangkefeng.wang@huawei.com>, Suren Baghdasaryan
 <surenb@google.com>
Subject: Re: [PATCH v2 2/2] mm/numa_balancing:Allow migrate on protnone
 reference with MPOL_PREFERRED_MANY policy
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On Mon, 11 Mar 2024 09:37:36 +0800 "Huang, Ying" <ying.huang@intel.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.