linux-kernel - Re: [PATCH 7/7] Add /proc/sys/vm/compact_node for the explicit compaction of a node

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Message-ID: <Pine.LNX.4.64.0705300920150.16108@skynet.skynet.ie>
Date:	Wed, 30 May 2007 09:26:40 +0100 (IST)
From:	Mel Gorman <mel@....ul.ie>
To:	Christoph Lameter <clameter@....com>
cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	kamezawa.hiroyu@...fujitsu.com
Subject: Re: [PATCH 7/7] Add /proc/sys/vm/compact_node for the explicit
 compaction of a node

On Tue, 29 May 2007, Christoph Lameter wrote:

> On Tue, 29 May 2007, Mel Gorman wrote:
>
>> +	if (nodeid < 0)
>> +		return -EINVAL;
>> +
>> +	pgdat = NODE_DATA(nodeid);
>> +	if (!pgdat || pgdat->node_id != nodeid)
>> +		return -EINVAL;
>
> You cannot pass an arbitrary number to node data since NODE_DATA may do a
> simple array lookup.
>
> Check for node < nr_node_ids first.
>

Very good point. Will fix

> pgdat->node_id != nodeid? Sounds like something you should BUG() on.
>

On non-NUMA, NODE_DATA(anything) returns contig_page_data. I was catching 
where the node ID's didn't match up because node 0 was always returned. 
Checking nr_node_ids is the correct way of doing this.

It's not a BUG() if bad ID is passed in here because we're checking user 
input. By returning -EINVAL the proc writer knows something bad happened 
without making a big deal about it.

> IA64's NODE_DATA is
>
> struct ia64_node_data {
>        short                   active_cpu_count;
>        short                   node;
>        struct pglist_data      *pg_data_ptrs[MAX_NUMNODES];
> };
>
> /*
> * Given a node id, return a pointer to the pg_data_t for the node.
> *
> * NODE_DATA    - should be used in all code not related to system
> *                initialization. It uses pernode data structures to minimize
> *                offnode memory references. However, these structure are not
> *                present during boot. This macro can be used once cpu_init
> *                completes.
> */
> #define NODE_DATA(nid)          (local_node_data->pg_data_ptrs[nid])
>
> x86_64 also does
>
> #define NODE_DATA(nid)          (node_data[nid])
>

All spot on. Will fix.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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