Re: [PATCH v6 1/6] mm/mempolicy: Add MPOL_PREFERRED_MANY for multiple preferred nodes

[Feng Tang]

On Mon, Aug 02, 2021 at 01:14:29PM +0200, Michal Hocko wrote:
> On Mon 02-08-21 16:11:30, Feng Tang wrote:
> > On Fri, Jul 30, 2021 at 03:18:40PM +0800, Tang, Feng wrote:
> > [snip]
> > > > > One thing is, it's possible that 'nd' is not set in the preferred
> > > > > nodemask. 
> > > > 
> > > > Yes, and there shouldn't be any problem with that.  The given node is
> > > > only used to get the respective zonelist (order distance ordered list of
> > > > zones to try). get_page_from_freelist will then use the preferred node
> > > > mask to filter this zone list. Is that more clear now?
> > > 
> > > Yes, from the code, the policy_node() is always coupled with
> > > policy_nodemask(), which secures the 'nodemask' limit. Thanks for
> > > the clarification!
> > 
> > Hi Michal,
> > 
> > To ensure the nodemask limit, the policy_nodemask() also needs some
> > change to return the nodemask for 'prefer-many' policy, so here is a
> > updated 1/6 patch, which mainly changes the node/nodemask selection
> > for 'prefer-many' policy, could you review it? thanks!
> 
> right, I have mixed it with get_policy_nodemask
> 
> > @@ -1875,8 +1897,13 @@ static int apply_policy_zone(struct mempolicy *policy, enum zone_type zone)
> >   */
> >  nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *policy)
> >  {
> > -	/* Lower zones don't get a nodemask applied for MPOL_BIND */
> > -	if (unlikely(policy->mode == MPOL_BIND) &&
> > +	int mode = policy->mode;
> > +
> > +	/*
> > +	 * Lower zones don't get a nodemask applied for 'bind' and
> > +	 * 'prefer-many' policies
> > +	 */
> > +	if (unlikely(mode == MPOL_BIND || mode == MPOL_PREFERRED_MANY) &&
> >  			apply_policy_zone(policy, gfp_zone(gfp)) &&
> >  			cpuset_nodemask_valid_mems_allowed(&policy->nodes))
> >  		return &policy->nodes;
> 
> Isn't this just too cryptic? Why didn't you simply
> 	if (mode == MPOL_PREFERRED_MANY)
> 		return &policy->mode;
> 
> in addition to the existing code? I mean why would you even care about
> cpusets? Those are handled at the page allocator layer and will further
> filter the given nodemask. 

Ok, I will follow your suggestion and keep 'bind' handling unchanged.

And to be honest, I don't fully understand the current handling for
'bind' policy, will the returning NULL for 'bind' policy open a
sideway for the strict 'bind' limit. 

Thanks,
Feng


> -- 
> Michal Hocko
> SUSE Labs