From 5d8c67ee11fb04059798541b0993803559c5f7af Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Sat, 10 Sep 2016 20:34:05 +1000 Subject: [PATCH] mm, compaction: create compact_gap wrapper Compaction uses a watermark gap of (2UL << order) pages at various places and it's not immediately obvious why. Abstract it through a compact_gap() wrapper to create a single place with a thorough explanation. Link: http://lkml.kernel.org/r/20160810091226.6709-9-vbabka@suse.cz Signed-off-by: Vlastimil Babka Acked-by: Michal Hocko Cc: Mel Gorman Cc: Joonsoo Kim Cc: David Rientjes Cc: Rik van Riel Signed-off-by: Andrew Morton --- include/linux/compaction.h | 16 ++++++++++++++++ mm/compaction.c | 7 +++---- mm/vmscan.c | 6 +++--- 3 files changed, 22 insertions(+), 7 deletions(-) diff --git a/include/linux/compaction.h b/include/linux/compaction.h index a1fba9994728..e7f0d34a90fe 100644 --- a/include/linux/compaction.h +++ b/include/linux/compaction.h @@ -58,6 +58,22 @@ enum compact_result { struct alloc_context; /* in mm/internal.h */ +/* + * Number of free order-0 pages that should be available above given watermark + * to make sure compaction has reasonable chance of not running out of free + * pages that it needs to isolate as migration target during its work. + */ +static inline unsigned long compact_gap(unsigned int order) +{ + /* + * Although all the isolations for migration are temporary, compaction + * may have up to 1 << order pages on its list and then try to split + * an (order - 1) free page. At that point, a gap of 1 << order might + * not be enough, so it's safer to require twice that amount. + */ + return 2UL << order; +} + #ifdef CONFIG_COMPACTION extern int sysctl_compact_memory; extern int sysctl_compaction_handler(struct ctl_table *table, int write, diff --git a/mm/compaction.c b/mm/compaction.c index e2618ac062a6..bbf41ee99142 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -1391,11 +1391,10 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order, return COMPACT_SUCCESS; /* - * Watermarks for order-0 must be met for compaction. Note the 2UL. - * This is because during migration, copies of pages need to be - * allocated and for a short time, the footprint is higher + * Watermarks for order-0 must be met for compaction to be able to + * isolate free pages for migration targets. */ - watermark = low_wmark_pages(zone) + (2UL << order); + watermark = low_wmark_pages(zone) + compact_gap(order); if (!__zone_watermark_ok(zone, 0, watermark, classzone_idx, alloc_flags, wmark_target)) return COMPACT_SKIPPED; diff --git a/mm/vmscan.c b/mm/vmscan.c index f7f5eb4b9d6f..9d5f80b714f4 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2499,7 +2499,7 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat, * If we have not reclaimed enough pages for compaction and the * inactive lists are large enough, continue reclaiming */ - pages_for_compaction = (2UL << sc->order); + pages_for_compaction = compact_gap(sc->order); inactive_lru_pages = node_page_state(pgdat, NR_INACTIVE_FILE); if (get_nr_swap_pages() > 0) inactive_lru_pages += node_page_state(pgdat, NR_INACTIVE_ANON); @@ -2631,7 +2631,7 @@ static inline bool compaction_ready(struct zone *zone, struct scan_control *sc) * there is a buffer of free pages available to give compaction * a reasonable chance of completing and allocating the page */ - watermark = high_wmark_pages(zone) + (2UL << sc->order); + watermark = high_wmark_pages(zone) + compact_gap(sc->order); watermark_ok = zone_watermark_ok_safe(zone, 0, watermark, sc->reclaim_idx); /* @@ -3188,7 +3188,7 @@ static bool kswapd_shrink_node(pg_data_t *pgdat, * excessive reclaim. Assume that a process requested a high-order * can direct reclaim/compact. */ - if (sc->order && sc->nr_reclaimed >= 2UL << sc->order) + if (sc->order && sc->nr_reclaimed >= compact_gap(sc->order)) sc->order = 0; return sc->nr_scanned >= sc->nr_to_reclaim; -- 2.34.1