Re: [patch] mm, compaction: properly initialize alloc_flags in compact_control

[Andrew Morton]

On Mon, 16 Oct 2017 15:03:37 -0700 (PDT) David Rientjes <rientjes@xxxxxxxxxx> 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.