mm: memcg: normalize the value passed into memcg_rstat_updated()

memcg_rstat_updated() uses the value of the state update to keep track of
the magnitude of pending updates, so that we only do a stats flush when
it's worth the work. Most values passed into memcg_rstat_updated() are in
pages, however, a few of them are actually in bytes or KBs.

To put this into perspective, a 512 byte slab allocation today would look
the same as allocating 512 pages. This may result in premature flushes,
which means unnecessary work and latency.

Normalize all the state values passed into memcg_rstat_updated() to pages.
Round up non-zero sub-page to 1 page, because memcg_rstat_updated()
ignores 0 page updates.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 5b3be698a872 ("memcg: better bounds on the memcg stats updates")
Signed-off-by: Yosry Ahmed <[e-mail]>
Acked-by: Johannes Weiner <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Michal Koutný <[e-mail]>
Cc: Muchun Song <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Cc: Shakeel Butt <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/memcontrol.c

···
 	return x;
 }
 
+static int memcg_page_state_unit(int item);
+
+/*
+ * Normalize the value passed into memcg_rstat_updated() to be in pages. Round
+ * up non-zero sub-page updates to 1 page as zero page updates are ignored.
+ */
+static int memcg_state_val_in_pages(int idx, int val)
+{
+	int unit = memcg_page_state_unit(idx);
+
+	if (!val || unit == PAGE_SIZE)
+		return val;
+	else
+		return max(val * unit / PAGE_SIZE, 1UL);
+}
+
 /**
  * __mod_memcg_state - update cgroup memory statistics
  * @memcg: the memory cgroup
···
 		return;
 
 	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
-	memcg_rstat_updated(memcg, val);
+	memcg_rstat_updated(memcg, memcg_state_val_in_pages(idx, val));
 }
 
 /* idx can be of type enum memcg_stat_item or node_stat_item. */
···
 	/* Update lruvec */
 	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
 
-	memcg_rstat_updated(memcg, val);
+	memcg_rstat_updated(memcg, memcg_state_val_in_pages(idx, val));
 	memcg_stats_unlock();
 }