Re: [RFC 09/13] mm, compaction: make whole_zone flag ignore cached scanner positions

[Michal Hocko]

On Tue 10-05-16 09:35:59, Vlastimil Babka wrote:
> A recent patch has added whole_zone flag that compaction sets when scanning
> starts from the zone boundary, in order to report that zone has been fully
> scanned in one attempt. For allocations that want to try really hard or cannot
> fail, we will want to introduce a mode where scanning whole zone is guaranteed
> regardless of the cached positions.
> 
> This patch reuses the whole_zone flag in a way that if it's already passed true
> to compaction, the cached scanner positions are ignored. Employing this flag
> during reclaim/compaction loop will be done in the next patch. This patch
> however converts compaction invoked from userspace via procfs to use this flag.
> Before this patch, the cached positions were first reset to zone boundaries and
> then read back from struct zone, so there was a window where a parallel
> compaction could replace the reset values, making the manual compaction less
> effective. Using the flag instead of performing reset is more robust.

This makes perfect sense because user visible API should better have a
deterministic behavior. Playing scanner positions games which might race
with the system triggered compaction sounds quite unpredictable.

> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  mm/compaction.c | 15 +++++----------
>  mm/internal.h   |  2 +-
>  2 files changed, 6 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index f649c7bc6de5..1ce6783d3ead 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1442,11 +1442,13 @@ static enum compact_result compact_zone(struct zone *zone, struct compact_contro
>  	 */
>  	cc->migrate_pfn = zone->compact_cached_migrate_pfn[sync];
>  	cc->free_pfn = zone->compact_cached_free_pfn;
> -	if (cc->free_pfn < start_pfn || cc->free_pfn >= end_pfn) {
> +	if (cc->whole_zone || cc->free_pfn < start_pfn ||
> +						cc->free_pfn >= end_pfn) {
>  		cc->free_pfn = pageblock_start_pfn(end_pfn - 1);
>  		zone->compact_cached_free_pfn = cc->free_pfn;
>  	}
> -	if (cc->migrate_pfn < start_pfn || cc->migrate_pfn >= end_pfn) {
> +	if (cc->whole_zone || cc->migrate_pfn < start_pfn ||
> +						cc->migrate_pfn >= end_pfn) {
>  		cc->migrate_pfn = start_pfn;
>  		zone->compact_cached_migrate_pfn[0] = cc->migrate_pfn;
>  		zone->compact_cached_migrate_pfn[1] = cc->migrate_pfn;
> @@ -1693,14 +1695,6 @@ static void __compact_pgdat(pg_data_t *pgdat, struct compact_control *cc)
>  		INIT_LIST_HEAD(&cc->freepages);
>  		INIT_LIST_HEAD(&cc->migratepages);
>  
> -		/*
> -		 * When called via /proc/sys/vm/compact_memory
> -		 * this makes sure we compact the whole zone regardless of
> -		 * cached scanner positions.
> -		 */
> -		if (is_via_compact_memory(cc->order))
> -			__reset_isolation_suitable(zone);
> -
>  		if (is_via_compact_memory(cc->order) ||
>  				!compaction_deferred(zone, cc->order))
>  			compact_zone(zone, cc);
> @@ -1736,6 +1730,7 @@ static void compact_node(int nid)
>  		.order = -1,
>  		.mode = MIGRATE_SYNC,
>  		.ignore_skip_hint = true,
> +		.whole_zone = true,
>  	};
>  
>  	__compact_pgdat(NODE_DATA(nid), &cc);
> diff --git a/mm/internal.h b/mm/internal.h
> index 556bc9d0a817..2acdee8ab0e6 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -178,7 +178,7 @@ struct compact_control {
>  	enum migrate_mode mode;		/* Async or sync migration mode */
>  	bool ignore_skip_hint;		/* Scan blocks even if marked skip */
>  	bool direct_compaction;		/* False from kcompactd or /proc/... */
> -	bool whole_zone;		/* Whole zone has been scanned */
> +	bool whole_zone;		/* Whole zone should/has been scanned */
>  	int order;			/* order a direct compactor needs */
>  	const gfp_t gfp_mask;		/* gfp mask of a direct compactor */
>  	const unsigned int alloc_flags;	/* alloc flags of a direct compactor */
> -- 
> 2.8.2
> 
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-- 
Michal Hocko
SUSE Labs