selftests: cgroup: refactor proactive reclaim code to reclaim_until()

Refactor the code that drives writing to memory.reclaim (retrying, error
handling, etc) from test_memcg_reclaim() to a helper called
reclaim_until(), which proactively reclaims from a memcg until its usage
reaches a certain value.

While we are at it, refactor and simplify the reclaim loop.

This will be used in a following patch in another test.

Link: https://lkml.kernel.org/r/20221202031512.1365483-3-yosryahmed@google.comSigned-off-by: Yosry Ahmed <yosryahmed@google.com>
Suggested-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Chris Down <chris@chrisdown.name>
Cc: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vasily Averin <vasily.averin@linux.dev>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/cgroup/test_memcontrol.c

@@ -645,6 +645,48 @@ static int test_memcg_max(const char *root)
 	return ret;
 }
 
+/*
+ * Reclaim from @memcg until usage reaches @goal by writing to
+ * memory.reclaim.
+ *
+ * This function will return false if the usage is already below the
+ * goal.
+ *
+ * This function assumes that writing to memory.reclaim is the only
+ * source of change in memory.current (no concurrent allocations or
+ * reclaim).
+ *
+ * This function makes sure memory.reclaim is sane. It will return
+ * false if memory.reclaim's error codes do not make sense, even if
+ * the usage goal was satisfied.
+ */
+static bool reclaim_until(const char *memcg, long goal)
+{
+	char buf[64];
+	int retries, err;
+	long current, to_reclaim;
+	bool reclaimed = false;
+
+	for (retries = 5; retries > 0; retries--) {
+		current = cg_read_long(memcg, "memory.current");
+
+		if (current < goal || values_close(current, goal, 3))
+			break;
+		/* Did memory.reclaim return 0 incorrectly? */
+		else if (reclaimed)
+			return false;
+
+		to_reclaim = current - goal;
+		snprintf(buf, sizeof(buf), "%ld", to_reclaim);
+		err = cg_write(memcg, "memory.reclaim", buf);
+		if (!err)
+			reclaimed = true;
+		else if (err != -EAGAIN)
+			return false;
+	}
+	return reclaimed;
+}
+
 /*
  * This test checks that memory.reclaim reclaims the given
  * amount of memory (from both anon and file, if possible).
@@ -653,8 +695,7 @@ static int test_memcg_reclaim(const char *root)
 {
 	int ret = KSFT_FAIL, fd, retries;
 	char *memcg;
-	long current, expected_usage, to_reclaim;
-	char buf[64];
+	long current, expected_usage;
 
 	memcg = cg_name(root, "memcg_test");
 	if (!memcg)
@@ -705,41 +746,8 @@ static int test_memcg_reclaim(const char *root)
 	 * Reclaim until current reaches 30M, this makes sure we hit both anon
 	 * and file if swap is enabled.
 	 */
-	retries = 5;
-	while (true) {
-		int err;
-
-		current = cg_read_long(memcg, "memory.current");
-		to_reclaim = current - MB(30);
-
-		/*
-		 * We only keep looping if we get EAGAIN, which means we could
-		 * not reclaim the full amount.
-		 */
-		if (to_reclaim <= 0)
-			goto cleanup;
-
-
-		snprintf(buf, sizeof(buf), "%ld", to_reclaim);
-		err = cg_write(memcg, "memory.reclaim", buf);
-		if (!err) {
-			/*
-			 * If writing succeeds, then the written amount should have been
-			 * fully reclaimed (and maybe more).
-			 */
-			current = cg_read_long(memcg, "memory.current");
-			if (!values_close(current, MB(30), 3) && current > MB(30))
-				goto cleanup;
-			break;
-		}
-
-		/* The kernel could not reclaim the full amount, try again. */
-		if (err == -EAGAIN && retries--)
-			continue;
-
-		/* We got an unexpected error or ran out of retries. */
+	if (!reclaim_until(memcg, MB(30)))
 		goto cleanup;
-	}
 
 	ret = KSFT_PASS;
 cleanup: