Re: [PATCH v1 4/4] mm/mempolicy: kill MPOL_F_LOCAL bit

[Feng Tang]

On Thu, May 27, 2021 at 10:20:08AM +0200, Michal Hocko wrote:
> On Wed 26-05-21 13:01:42, Feng Tang wrote:
> > Now the only remaining case of a real 'local' policy faked by
> > 'prefer' policy plus MPOL_F_LOCAL bit is:
> > 
> > A valid 'prefer' policy with a valid 'preferred' node is 'rebind'
> > to a nodemask which doesn't contains the 'preferred' node, then it
> > will handle allocation with 'local' policy.
> > 
> > Add a new 'MPOL_F_LOCAL_TEMP' bit for this case, and kill the
> > MPOL_F_LOCAL bit, which could simplify the code much.
> 
> As I've pointed out in the reply to the previous patch. It would have
> been much better if most of the MPOL_F_LOCAL usage was gone by this
> patch.
> 
> I also dislike a new MPOL_F_LOCAL_TEMP. This smells like sneaking the
> hack back in after you have painstakingly removed it. So this looks like
> a step backwards to me. I also do not understand why do we need the
> rebind callback for local policy at all. There is no node mask for local
> so what is going on here?

This is the special case 4 for 'perfer' policy with MPOL_F_STATIC_NODES
flag set, say it prefer node 1, when it is later 'refind' to a new
nodemask node 2-3, according to current code it will be add the
MPOL_F_LOCAL bit and performs 'local' policy acctually. And in future
it is 'rebind' again with a nodemask 1-2, it will be restored back
to 'prefer' policy with preferred node 1.

This patch tries to address this special case. I have struggled but 
couldn't think of a good way. Any suggestions? thanks!

- Feng

> [...]
> > +static void mpol_rebind_local(struct mempolicy *pol,
> > +				const nodemask_t *nodes)
> > +{
> > +	if (unlikely(pol->flags & MPOL_F_STATIC_NODES)) {
> > +		int node = first_node(pol->w.user_nodemask);
> > +
> > +		BUG_ON(!(pol->flags & MPOL_F_LOCAL_TEMP));
> > +
> > +		if (node_isset(node, *nodes)) {
> > +			pol->v.preferred_node = node;
> > +			pol->mode = MPOL_PREFERRED;
> > +			pol->flags &= ~MPOL_F_LOCAL_TEMP;
> > +		}
> > +	}
> > +}
> > +
> 
> I have to confess I've got lost here. Could you explain why do you need
> all this handling for a local policy?
> 
> >  static void mpol_rebind_preferred(struct mempolicy *pol,
> >  						const nodemask_t *nodes)
> >  {
> > @@ -347,13 +363,19 @@ static void mpol_rebind_preferred(struct mempolicy *pol,
> >  
> >  		if (node_isset(node, *nodes)) {
> >  			pol->v.preferred_node = node;
> > -			pol->flags &= ~MPOL_F_LOCAL;
> > -		} else
> > -			pol->flags |= MPOL_F_LOCAL;
> > +		} else {
> > +			/*
> > +			 * If there is no valid node, change the mode to
> > +			 * MPOL_LOCAL, which will be restored back when
> > +			 * next rebind() sees a valid node.
> > +			 */
> > +			pol->mode = MPOL_LOCAL;
> > +			pol->flags |= MPOL_F_LOCAL_TEMP;
> > +		}
> >  	} else if (pol->flags & MPOL_F_RELATIVE_NODES) {
> >  		mpol_relative_nodemask(&tmp, &pol->w.user_nodemask, nodes);
> >  		pol->v.preferred_node = first_node(tmp);
> > -	} else if (!(pol->flags & MPOL_F_LOCAL)) {
> > +	} else {
> >  		pol->v.preferred_node = node_remap(pol->v.preferred_node,
> >  						   pol->w.cpuset_mems_allowed,
> >  						   *nodes);
> > @@ -372,7 +394,7 @@ static void mpol_rebind_policy(struct mempolicy *pol, const nodemask_t *newmask)
> >  {
> >  	if (!pol)
> >  		return;
> > -	if (!mpol_store_user_nodemask(pol) && !(pol->flags & MPOL_F_LOCAL) &&
> > +	if (!mpol_store_user_nodemask(pol) &&
> >  	    nodes_equal(pol->w.cpuset_mems_allowed, *newmask))
> >  		return;
> >  
> > @@ -425,7 +447,7 @@ static const struct mempolicy_operations mpol_ops[MPOL_MAX] = {
> >  	},
> >  	[MPOL_LOCAL] = {
> >  		.create = mpol_new_local,
> > -		.rebind = mpol_rebind_default,
> > +		.rebind = mpol_rebind_local,
> >  	},
> >  };
> -- 
> Michal Hocko
> SUSE Labs