mm, vmstat: remove zone and node double accounting by approximating retries

The number of LRU pages, dirty pages and writeback pages must be
accounted for on both zones and nodes because of the reclaim retry
logic, compaction retry logic and highmem calculations all depending on
per-zone stats.

Many lowmem allocations are immune from OOM kill due to a check in
__alloc_pages_may_oom for (ac->high_zoneidx < ZONE_NORMAL) since commit
03668b3ceb0c ("oom: avoid oom killer for lowmem allocations").  The
exception is costly high-order allocations or allocations that cannot
fail.  If the __alloc_pages_may_oom avoids OOM-kill for low-order lowmem
allocations then it would fall through to __alloc_pages_direct_compact.

This patch will blindly retry reclaim for zone-constrained allocations
in should_reclaim_retry up to MAX_RECLAIM_RETRIES.  This is not ideal
but without per-zone stats there are not many alternatives.  The impact
it that zone-constrained allocations may delay before considering the
OOM killer.

As there is no guarantee enough memory can ever be freed to satisfy
compaction, this patch avoids retrying compaction for zone-contrained
allocations.

In combination, that means that the per-node stats can be used when
deciding whether to continue reclaim using a rough approximation.  While
it is possible this will make the wrong decision on occasion, it will
not infinite loop as the number of reclaim attempts is capped by
MAX_RECLAIM_RETRIES.

The final step is calculating the number of dirtyable highmem pages.  As
those calculations only care about the global count of file pages in
highmem.  This patch uses a global counter used instead of per-zone
stats as it is sufficient.

In combination, this allows the per-zone LRU and dirty state counters to
be removed.

[mgorman@techsingularity.net: fix acct_highmem_file_pages()]
  Link: http://lkml.kernel.org/r/1468853426-12858-4-git-send-email-mgorman@techsingularity.netLink: http://lkml.kernel.org/r/1467970510-21195-35-git-send-email-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Suggested by: Michal Hocko <mhocko@kernel.org>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 19 insertions, 1 deletions

diff --git a/mm/compaction.c b/mm/compaction.c
index e5995f38d677..cd93ea24c565 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1438,6 +1438,11 @@ bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
 {
 	struct zone *zone;
 	struct zoneref *z;
+	pg_data_t *last_pgdat = NULL;
+
+	/* Do not retry compaction for zone-constrained allocations */
+	if (ac->high_zoneidx < ZONE_NORMAL)
+		return false;
 
 	/*
 	 * Make sure at least one zone would pass __compaction_suitable if we continue
@@ -1448,14 +1453,27 @@ bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
 		unsigned long available;
 		enum compact_result compact_result;
 
+		if (last_pgdat == zone->zone_pgdat)
+			continue;
+
+		/*
+		 * This over-estimates the number of pages available for
+		 * reclaim/compaction but walking the LRU would take too
+		 * long. The consequences are that compaction may retry
+		 * longer than it should for a zone-constrained allocation
+		 * request.
+		 */
+		last_pgdat = zone->zone_pgdat;
+		available = pgdat_reclaimable_pages(zone->zone_pgdat) / order;
+
 		/*
 		 * Do not consider all the reclaimable memory because we do not
 		 * want to trash just for a single high order allocation which
 		 * is even not guaranteed to appear even if __compaction_suitable
 		 * is happy about the watermark check.
 		 */
-		available = zone_reclaimable_pages(zone) / order;
 		available += zone_page_state_snapshot(zone, NR_FREE_PAGES);
+		available = min(zone->managed_pages, available);
 		compact_result = __compaction_suitable(zone, order, alloc_flags,
 				ac_classzone_idx(ac), available);
 		if (compact_result != COMPACT_SKIPPED &&