mm, vmscan: account for skipped pages as a partial scan

Page reclaim determines whether a pgdat is unreclaimable by examining
how many pages have been scanned since a page was freed and comparing
that to the LRU sizes.  Skipped pages are not reclaim candidates but
contribute to scanned.  This can prematurely mark a pgdat as
unreclaimable and trigger an OOM kill.

This patch accounts for skipped pages as a partial scan so that an
unreclaimable pgdat will still be marked as such but by scaling the cost
of a skip, it'll avoid the pgdat being marked prematurely.

Link: http://lkml.kernel.org/r/1469110261-7365-6-git-send-email-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -1424,7 +1424,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 	LIST_HEAD(pages_skipped);
 
 	for (scan = 0; scan < nr_to_scan && nr_taken < nr_to_scan &&
-					!list_empty(src); scan++) {
+					!list_empty(src);) {
 		struct page *page;
 
 		page = lru_to_page(src);
@@ -1438,6 +1438,12 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 			continue;
 		}
 
+		/*
+		 * Account for scanned and skipped separetly to avoid the pgdat
+		 * being prematurely marked unreclaimable by pgdat_reclaimable.
+		 */
+		scan++;
+
 		switch (__isolate_lru_page(page, mode)) {
 		case 0:
 			nr_pages = hpage_nr_pages(page);
@@ -1465,14 +1471,24 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 	 */
 	if (!list_empty(&pages_skipped)) {
 		int zid;
+		unsigned long total_skipped = 0;
 
-		list_splice(&pages_skipped, src);
 		for (zid = 0; zid < MAX_NR_ZONES; zid++) {
 			if (!nr_skipped[zid])
 				continue;
 
 			__count_zid_vm_events(PGSCAN_SKIP, zid, nr_skipped[zid]);
+			total_skipped += nr_skipped[zid];
 		}
+
+		/*
+		 * Account skipped pages as a partial scan as the pgdat may be
+		 * close to unreclaimable. If the LRU list is empty, account
+		 * skipped pages as a full scan.
+		 */
+		scan += list_empty(src) ? total_skipped : total_skipped >> 2;
+
+		list_splice(&pages_skipped, src);
 	}
 	*nr_scanned = scan;
 	trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan, scan,