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

From: Michal Hocko <mhocko@suse.com>
To: Feng Tang <feng.tang@intel.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	David Rientjes <rientjes@google.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Ben Widawsky <ben.widawsky@intel.com>,
	linux-kernel@vger.kernel.org,
	Andrea Arcangeli <aarcange@redhat.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	Vlastimil Babka <vbabka@suse.cz>, Andi Kleen <ak@linux.intel.com>,
	Dan Williams <dan.j.williams@intel.com>,
	ying.huang@intel.com
Subject: Re: [PATCH v1 4/4] mm/mempolicy: kill MPOL_F_LOCAL bit
Date: Thu, 27 May 2021 10:20:08 +0200	[thread overview]
Message-ID: <YK9WOKBRsaFESPfR@dhcp22.suse.cz> (raw)
In-Reply-To: <1622005302-23027-5-git-send-email-feng.tang@intel.com>

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?

[...]
> +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