linux-kernel - Re: [PATCH v6 1/6] mm/mempolicy: Add MPOL_PREFERRED

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Date:   Fri, 30 Jul 2021 15:18:40 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ben Widawsky <ben.widawsky@...el.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andi Kleen <ak@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>, ying.huang@...el.com,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v6 1/6] mm/mempolicy: Add MPOL_PREFERRED_MANY for
 multiple preferred nodes

On Fri, Jul 30, 2021 at 08:36:50AM +0200, Michal Hocko wrote:
> On Fri 30-07-21 11:05:02, Feng Tang wrote:
> > On Thu, Jul 29, 2021 at 06:21:19PM +0200, Michal Hocko wrote:
> > > On Thu 29-07-21 23:12:42, Feng Tang wrote:
> > > > On Thu, Jul 29, 2021 at 03:38:44PM +0200, Michal Hocko wrote:
> > > [...]
> > > > > Also the
> > > > > semantic to give nodes some ordering based on their numbers sounds
> > > > > rather weird to me.
> > > > 
> > > > I agree, and as I admitted in the first reply, this need to be fixed.
> > > 
> > > OK. I was not really clear that we are on the same page here.
> > > 
> > > > > The semantic I am proposing is to allocate from prefered nodes in
> > > > > distance order starting from the local node.
> > > > 
> > > > So the plan is:
> > > > * if the local node is set in 'prefer-many's nodemask, then chose
> > > > * otherwise chose the node with the shortest distance to local node
> > > > ?
> > > 
> > > Yes and what I am trying to say is that you will achieve that simply by
> > > doing the following in policy_node:
> > > 	if (policy->mode == MPOL_PREFERRED_MANY)
> > > 		return nd;
> > 
> > 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!

And for the mempolicy_slab_node(), it seems to be a little different,
and we may need to reuse its logic for 'bind' policy, which is similar
to what we've discussed, pick a nearest node to the local node. And
similar for mpol_misplaced(). Thoughts?

Thanks,
Feng

> -- 
> Michal Hocko
> SUSE Labs