Re: [PATCH v2 2/2] mm/numa_balancing:Allow migrate on protnone reference with MPOL_PREFERRED_MANY policy

[Andrew Morton]

On Mon, 11 Mar 2024 09:37:36 +0800 "Huang, Ying" <ying.huang@xxxxxxxxx> 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.