On Fri, Jul 14, 2017 at 03:02:42PM +0200, Michal Hocko wrote:
> > It *might* be safer given the next patch to zero out the remainder of
> > the _zonerefs to that there is no combination of node add/remove that has
> > an iterator working with a semi-valid _zoneref which is beyond the last
> > correct value. It *should* be safe as the very last entry will always
> > be null but if you don't zero it out, it is possible for iterators to be
> > working beyond the "end" of the zonelist for a short window.
>
> yes that is true but there will always be terminating NULL zone and I
> found that acceptable. It is basically the same thing as accessing an
> empty zone or a zone twice. Or do you think this is absolutely necessary
> to handle?
>
I don't think it's absolutely necessary. While you could construct some
odd behaviour for iterators currently past the end of the list, they would
eventually encounter a NULL.
> > Otherwise think it's ok including my stupid comment about node_order
> > stack usage.
>
> What do you think about this on top?
> ---
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 49bade7ff049..3b98524c04ec 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4913,20 +4913,21 @@ static int find_next_best_node(int node, nodemask_t *used_node_mask)
> * This results in maximum locality--normal zone overflows into local
> * DMA zone, if any--but risks exhausting DMA zone.
> */
> -static void build_zonelists_in_node_order(pg_data_t *pgdat, int *node_order)
> +static void build_zonelists_in_node_order(pg_data_t *pgdat, int *node_order,
> + unsigned nr_nodes)
> {
> struct zonelist *zonelist;
> - int i, zoneref_idx = 0;
> + int i, nr_zones = 0;
>
> zonelist = &pgdat->node_zonelists[ZONELIST_FALLBACK];
>
> - for (i = 0; i < MAX_NUMNODES; i++) {
> + for (i = 0; i < nr_nodes; i++) {
The first iteration is then -- for (i = 0; i < 0; i++)
Fairly sure that's not what you meant.
> pg_data_t *node = NODE_DATA(node_order[i]);
>
> - zoneref_idx = build_zonelists_node(node, zonelist, zoneref_idx);
> + nr_zones = build_zonelists_node(node, zonelist, nr_zones);
I meant converting build_zonelists_node and passing in &nr_zones and
returning false when an empty node is encountered. In this context,
it's also not about zones, it really is nr_zonerefs. Rename nr_zones in
build_zonelists_node as well.
--
Mel Gorman
SUSE Labs
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