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

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Date:   Mon, 2 Aug 2021 13:14:29 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Widawsky, Ben" <ben.widawsky@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-api@...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>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "Huang, Ying" <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 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. 

-- 
Michal Hocko
SUSE Labs