mm: memcg: rename and document global_reclaim()

Evidently, global_reclaim() can be a confusing name.  Especially that it
used to exist before with a subtly different definition (removed by commit
b5ead35 ("mm: vmscan: naming fixes: global_reclaim() and
sane_reclaim()").  It can be interpreted as non-cgroup reclaim, even
though it returns true for cgroup reclaim on the root memcg (through
memory.reclaim).

Rename it to root_reclaim() in an attempt to make it less ambiguous, and
add documentation to it as well as cgroup_reclaim.

Link: https://lkml.kernel.org/r/20230621023053.432374-1-yosryahmed@google.com
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Reported-by: Johannes Weiner <hannes@cmpxchg.org>
Closes: https://lore.kernel.org/lkml/20230405200150.GA35884@cmpxchg.org/
Acked-by: Yu Zhao <yuzhao@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: yosrym93

mm/vmscan.c

@@ -429,12 +429,17 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
 	up_read(&shrinker_rwsem);
 }
 
+/* Returns true for reclaim through cgroup limits or cgroup interfaces. */
 static bool cgroup_reclaim(struct scan_control *sc)
 {
 	return sc->target_mem_cgroup;
 }
 
-static bool global_reclaim(struct scan_control *sc)
+/*
+ * Returns true for reclaim on the root cgroup. This is true for direct
+ * allocator reclaim and reclaim through cgroup interfaces on the root cgroup.
+ */
+static bool root_reclaim(struct scan_control *sc)
 {
 	return !sc->target_mem_cgroup || mem_cgroup_is_root(sc->target_mem_cgroup);
 }
@@ -489,7 +494,7 @@ static bool cgroup_reclaim(struct scan_control *sc)
 	return false;
 }
 
-static bool global_reclaim(struct scan_control *sc)
+static bool root_reclaim(struct scan_control *sc)
 {
 	return true;
 }
@@ -546,7 +551,7 @@ static void flush_reclaim_state(struct scan_control *sc)
 	 * memcg reclaim, to make reporting more accurate and reduce
 	 * underestimation, but it's probably not worth the complexity for now.
 	 */
-	if (current->reclaim_state && global_reclaim(sc)) {
+	if (current->reclaim_state && root_reclaim(sc)) {
 		sc->nr_reclaimed += current->reclaim_state->reclaimed;
 		current->reclaim_state->reclaimed = 0;
 	}
@@ -5325,7 +5330,7 @@ static long get_nr_to_scan(struct lruvec *lruvec, struct scan_control *sc, bool
 static unsigned long get_nr_to_reclaim(struct scan_control *sc)
 {
 	/* don't abort memcg reclaim to ensure fairness */
-	if (!global_reclaim(sc))
+	if (!root_reclaim(sc))
 		return -1;
 
 	return max(sc->nr_to_reclaim, compact_gap(sc->order));
@@ -5477,7 +5482,7 @@ static void lru_gen_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc
 {
 	struct blk_plug plug;
 
-	VM_WARN_ON_ONCE(global_reclaim(sc));
+	VM_WARN_ON_ONCE(root_reclaim(sc));
 	VM_WARN_ON_ONCE(!sc->may_writepage || !sc->may_unmap);
 
 	lru_add_drain();
@@ -5538,7 +5543,7 @@ static void lru_gen_shrink_node(struct pglist_data *pgdat, struct scan_control *
 	struct blk_plug plug;
 	unsigned long reclaimed = sc->nr_reclaimed;
 
-	VM_WARN_ON_ONCE(!global_reclaim(sc));
+	VM_WARN_ON_ONCE(!root_reclaim(sc));
 
 	/*
 	 * Unmapped clean folios are already prioritized. Scanning for more of
@@ -6260,7 +6265,7 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 	bool proportional_reclaim;
 	struct blk_plug plug;
 
-	if (lru_gen_enabled() && !global_reclaim(sc)) {
+	if (lru_gen_enabled() && !root_reclaim(sc)) {
 		lru_gen_shrink_lruvec(lruvec, sc);
 		return;
 	}
@@ -6501,7 +6506,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 	struct lruvec *target_lruvec;
 	bool reclaimable = false;
 
-	if (lru_gen_enabled() && global_reclaim(sc)) {
+	if (lru_gen_enabled() && root_reclaim(sc)) {
 		lru_gen_shrink_node(pgdat, sc);
 		return;
 	}