mm: don't avoid high-priority reclaim on memcg limit reclaim

Commit 246e87a9 ("memcg: fix get_scan_count() for small targets")
sought to avoid high reclaim priorities for memcg by forcing it to scan
a minimum amount of pages when lru_pages >> priority yielded nothing.
This was done at a time when reclaim decisions like dirty throttling
were tied to the priority level.

Nowadays, the only meaningful thing still tied to priority dropping
below DEF_PRIORITY - 2 is gating whether laptop_mode=1 is generally
allowed to write.  But that is from an era where direct reclaim was
still allowed to call ->writepage, and kswapd nowadays avoids writes
until it's scanned every clean page in the system.  Potential changes to
how quick sc->may_writepage could trigger are of little concern.

Remove the force_scan stuff, as well as the ugly multi-pass target
calculation that it necessitated.

Link: http://lkml.kernel.org/r/20170228214007.5621-7-hannes@cmpxchg.orgSigned-off-by: NJohannes Weiner <hannes@cmpxchg.org>
Acked-by: NHillf Danton <hillf.zj@alibaba-inc.com>
Acked-by: NMichal Hocko <mhocko@suse.com>
Cc: Jia He <hejianet@gmail.com>
Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -2123,21 +2123,8 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
 	unsigned long anon_prio, file_prio;
 	enum scan_balance scan_balance;
 	unsigned long anon, file;
-	bool force_scan = false;
 	unsigned long ap, fp;
 	enum lru_list lru;
-	bool some_scanned;
-	int pass;
-
-	/*
-	 * If the zone or memcg is small, nr[l] can be 0. When
-	 * reclaiming for a memcg, a priority drop can cause high
-	 * latencies, so it's better to scan a minimum amount. When a
-	 * cgroup has already been deleted, scrape out the remaining
-	 * cache forcefully to get rid of the lingering state.
-	 */
-	if (!global_reclaim(sc) || !mem_cgroup_online(memcg))
-		force_scan = true;
 
 	/* If we have no swap space, do not bother scanning anon pages. */
 	if (!sc->may_swap || mem_cgroup_get_nr_swap_pages(memcg) <= 0) {
@@ -2268,9 +2255,6 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
 	fraction[1] = fp;
 	denominator = ap + fp + 1;
 out:
-	some_scanned = false;
-	/* Only use force_scan on second pass. */
-	for (pass = 0; !some_scanned && pass < 2; pass++) {
 	*lru_pages = 0;
 	for_each_evictable_lru(lru) {
 		int file = is_file_lru(lru);
@@ -2279,8 +2263,11 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
 
 		size = lruvec_lru_size(lruvec, lru, sc->reclaim_idx);
 		scan = size >> sc->priority;
-
-			if (!scan && pass && force_scan)
+		/*
+		 * If the cgroup's already been deleted, make sure to
+		 * scrape out the remaining cache.
+		 */
+		if (!scan && !mem_cgroup_online(memcg))
 			scan = min(size, SWAP_CLUSTER_MAX);
 
 		switch (scan_balance) {
@@ -2310,13 +2297,6 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
 
 		*lru_pages += size;
 		nr[lru] = scan;
-
-			/*
-			 * Skip the second pass and don't force_scan,
-			 * if we found something to scan.
-			 */
-			some_scanned |= !!scan;
-		}
 	}
 }