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

Commit 246e87a93934 ("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.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Jia He <hejianet@gmail.com>
Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2fd50ca88016307e94a62dac864bc1c2523334eb..9117ae8d49eed25939fedcddfcc11c278bca6bd4 100644 (file)
--- a/mm/vmscan.c
+++ b/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) {
@@ -2268,55 +2255,48 @@ 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);
-                       unsigned long size;
-                       unsigned long scan;
-
-                       size = lruvec_lru_size(lruvec, lru, sc->reclaim_idx);
-                       scan = size >> sc->priority;
-
-                       if (!scan && pass && force_scan)
-                               scan = min(size, SWAP_CLUSTER_MAX);
-
-                       switch (scan_balance) {
-                       case SCAN_EQUAL:
-                               /* Scan lists relative to size */
-                               break;
-                       case SCAN_FRACT:
-                               /*
-                                * Scan types proportional to swappiness and
-                                * their relative recent reclaim efficiency.
-                                */
-                               scan = div64_u64(scan * fraction[file],
-                                                       denominator);
-                               break;
-                       case SCAN_FILE:
-                       case SCAN_ANON:
-                               /* Scan one type exclusively */
-                               if ((scan_balance == SCAN_FILE) != file) {
-                                       size = 0;
-                                       scan = 0;
-                               }
-                               break;
-                       default:
-                               /* Look ma, no brain */
-                               BUG();
-                       }
+       *lru_pages = 0;
+       for_each_evictable_lru(lru) {
+               int file = is_file_lru(lru);
+               unsigned long size;
+               unsigned long scan;
 
-                       *lru_pages += size;
-                       nr[lru] = scan;
+               size = lruvec_lru_size(lruvec, lru, sc->reclaim_idx);
+               scan = size >> sc->priority;
+               /*
+                * 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) {
+               case SCAN_EQUAL:
+                       /* Scan lists relative to size */
+                       break;
+               case SCAN_FRACT:
                        /*
-                        * Skip the second pass and don't force_scan,
-                        * if we found something to scan.
+                        * Scan types proportional to swappiness and
+                        * their relative recent reclaim efficiency.
                         */
-                       some_scanned |= !!scan;
+                       scan = div64_u64(scan * fraction[file],
+                                        denominator);
+                       break;
+               case SCAN_FILE:
+               case SCAN_ANON:
+                       /* Scan one type exclusively */
+                       if ((scan_balance == SCAN_FILE) != file) {
+                               size = 0;
+                               scan = 0;
+                       }
+                       break;
+               default:
+                       /* Look ma, no brain */
+                       BUG();
                }
+
+               *lru_pages += size;
+               nr[lru] = scan;
        }
 }