On Mon, 16 Oct 2017 15:03:37 -0700 (PDT) David Rientjes <[email protected]>
wrote:
>
> compaction_suitable() requires a useful cc->alloc_flags, otherwise the
> results of compact_zone() can be indeterminate. Kcompactd currently
> checks compaction_suitable() itself with alloc_flags == 0, but passes an
> uninitialized value from the stack to compact_zone(), which does its own
> check.
>
> The same is true for compact_node() when explicitly triggering full node
> compaction.
>
> Properly initialize cc.alloc_flags on the stack.
>
The compiler will zero any not-explicitly-initialized fields in these
initializers.
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1792,9 +1792,9 @@ static void compact_node(int nid)
> {
> pg_data_t *pgdat = NODE_DATA(nid);
> int zoneid;
> - struct zone *zone;
> struct compact_control cc = {
> .order = -1,
> + .alloc_flags = 0,
> .total_migrate_scanned = 0,
> .total_free_scanned = 0,
> .mode = MIGRATE_SYNC,
> @@ -1805,6 +1805,7 @@ static void compact_node(int nid)
>
>
> for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
> + struct zone *zone;
>
> zone = &pgdat->node_zones[zoneid];
> if (!populated_zone(zone))
> @@ -1923,6 +1924,7 @@ static void kcompactd_do_work(pg_data_t *pgdat)
> struct zone *zone;
> struct compact_control cc = {
> .order = pgdat->kcompactd_max_order,
> + .alloc_flags = 0,
> .total_migrate_scanned = 0,
> .total_free_scanned = 0,
> .classzone_idx = pgdat->kcompactd_classzone_idx,
> @@ -1945,8 +1947,8 @@ static void kcompactd_do_work(pg_data_t *pgdat)
> if (compaction_deferred(zone, cc.order))
> continue;
>
> - if (compaction_suitable(zone, cc.order, 0, zoneid) !=
> - COMPACT_CONTINUE)
> + if (compaction_suitable(zone, cc.order, cc.alloc_flags,
> + zoneid) != COMPACT_CONTINUE)
> continue;
So afaict the above hunk is the only functional change here. It will
propagate any of compact_zone()'s modifications to cc->alloc_flags into
succeeding calls to compaction_suitable(). I suspect this is a
no-op (didn't look), and it wasn't changelogged.
