HPA: Fix infinite purging loop

One of the condition to start purging is `hpa_hugify_blocked_by_ndirty`
function call returns true. This can happen in cases where we have no
dirty memory for this shard at all. In this case purging loop will be an
infinite loop.

`hpa_hugify_blocked_by_ndirty` was introduced at 0f6c420, but at that
time purging loop has different form and additional `break` was not
required. Purging loop form was re-written at 6630c59, but additional
exit condition wasn't added there at the time.

Repo code was shared by Patrik Dokoupil at [1], I stripped it down to
minimum to reproduce issue in jemalloc unit tests.

[1]: #2533

src/hpa.c

@@ -537,9 +537,16 @@ hpa_shard_maybe_do_deferred_work(tsdn_t *tsdn, hpa_shard_t *shard,
 		purged = false;
 		while (hpa_should_purge(tsdn, shard) && nops < max_ops) {
 			purged = hpa_try_purge(tsdn, shard);
-			if (purged) {
-				nops++;
+			if (!purged) {
+				/*
+				 * It is fine if we couldn't purge as sometimes
+				 * we try to purge just to unblock
+				 * hugification, but there is maybe no dirty
+				 * pages at all at the moment.
+				 */
+				break;
 			}
+			nops++;
 		}
 		hugified = hpa_try_hugify(tsdn, shard);
 		if (hugified) {

test/unit/hpa.c

@@ -24,7 +24,7 @@ struct test_data_s {
 static hpa_shard_opts_t test_hpa_shard_opts_default = {
 	/* slab_max_alloc */
 	ALLOC_MAX,
-	/* hugification threshold */
+	/* hugification_threshold */
 	HUGEPAGE,
 	/* dirty_mult */
 	FXP_INIT_PERCENT(25),
@@ -36,6 +36,21 @@ static hpa_shard_opts_t test_hpa_shard_opts_default = {
 	5 * 1000,
 };
 
+static hpa_shard_opts_t test_hpa_shard_opts_purge = {
+	/* slab_max_alloc */
+	HUGEPAGE,
+	/* hugification_threshold */
+	0.9 * HUGEPAGE,
+	/* dirty_mult */
+	FXP_INIT_PERCENT(11),
+	/* deferral_allowed */
+	true,
+	/* hugify_delay_ms */
+	0,
+	/* min_purge_interval_ms */
+	5 * 1000,
+};
+
 static hpa_shard_t *
 create_test_data(const hpa_hooks_t *hooks, hpa_shard_opts_t *opts) {
 	bool err;
@@ -452,6 +467,36 @@ TEST_BEGIN(test_defer_time) {
 }
 TEST_END
 
+TEST_BEGIN(test_purge_no_infinite_loop) {
+	test_skip_if(!hpa_supported());
+
+	hpa_shard_t *shard = create_test_data(&hpa_hooks_default,
+	    &test_hpa_shard_opts_purge);
+	tsdn_t *tsdn = tsd_tsdn(tsd_fetch());
+
+	/*
+	 * This is not arbitrary value, it is chosen to met hugification
+	 * criteria for huge page and at the same time do not allow hugify page
+	 * without triggering a purge.
+	 */
+	const size_t npages =
+	    test_hpa_shard_opts_purge.hugification_threshold / PAGE + 1;
+	const size_t size = npages * PAGE;
+
+	bool deferred_work_generated = false;
+	edata_t *edata = pai_alloc(tsdn, &shard->pai, size, PAGE,
+	     /* zero */ false, /* guarded */ false, /* frequent_reuse */ false,
+	     &deferred_work_generated);
+	expect_ptr_not_null(edata, "Unexpected alloc failure");
+
+	hpa_shard_do_deferred_work(tsdn, shard);
+
+	/* hpa_shard_do_deferred_work should not stuck in a purging loop */
+
+	destroy_test_data(shard);
+}
+TEST_END
+
 int
 main(void) {
 	/*
@@ -470,5 +515,6 @@ main(void) {
 	    test_alloc_max,
 	    test_stress,
 	    test_alloc_dalloc_batch,
-	    test_defer_time);
+	    test_defer_time,
+	    test_purge_no_infinite_loop);
 }