Re: [PATCH 4/11] ksm: reorganize ksm_check_stable_tree

[Hugh Dickins]

On Tue, 5 Feb 2013, Mel Gorman wrote:
> On Fri, Jan 25, 2013 at 05:59:35PM -0800, Hugh Dickins wrote:
> > Memory hotremove's ksm_check_stable_tree() is pitifully inefficient
> > (restarting whenever it finds a stale node to remove), but rearrange
> > so that at least it does not needlessly restart from nid 0 each time.
> > And add a couple of comments: here is why we keep pfn instead of page.
> > 
> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> > ---
> >  mm/ksm.c |   38 ++++++++++++++++++++++----------------
> >  1 file changed, 22 insertions(+), 16 deletions(-)
> > 
> > --- mmotm.orig/mm/ksm.c	2013-01-25 14:36:52.152205940 -0800
> > +++ mmotm/mm/ksm.c	2013-01-25 14:36:53.244205966 -0800
> > @@ -1830,31 +1830,36 @@ void ksm_migrate_page(struct page *newpa
> >  #endif /* CONFIG_MIGRATION */
> >  
> >  #ifdef CONFIG_MEMORY_HOTREMOVE
> > -static struct stable_node *ksm_check_stable_tree(unsigned long start_pfn,
> > -						 unsigned long end_pfn)
> > +static void ksm_check_stable_tree(unsigned long start_pfn,
> > +				  unsigned long end_pfn)
> >  {
> > +	struct stable_node *stable_node;
> >  	struct rb_node *node;
> >  	int nid;
> >  
> > -	for (nid = 0; nid < nr_node_ids; nid++)
> > -		for (node = rb_first(&root_stable_tree[nid]); node;
> > -				node = rb_next(node)) {
> > -			struct stable_node *stable_node;
> > -
> > +	for (nid = 0; nid < nr_node_ids; nid++) {
> > +		node = rb_first(&root_stable_tree[nid]);
> > +		while (node) {
> 
> This is not your fault, the old code is wrong too. It is assuming that all
> nodes are populated in numeric orders with no holes. It won't work if just
> two nodes 0 and 4 are online. It should be using for_each_online_node().

If the old code is wrong, it probably would be my fault!  But I believe
this is okay: these rb_roots we're looking at, they are in memory which
is not being offlined, and the trees for offline nodes will simply be
empty, won't they?  Something's badly wrong if otherwise.

I certainly prefer to avoid for_each_online_node() etc: maybe I'm
confusing with for_each_online_something_else(), but experience tells
that you can get into nasty hotplug mutex ordering issues with those
things - not worth the pain if you can easily and safely avoid them.

Hugh
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