diff --git a/include/jemalloc/internal/arena_externs.h b/include/jemalloc/internal/arena_externs.h
index 8dd5b015..91fed258 100644
--- a/include/jemalloc/internal/arena_externs.h
+++ b/include/jemalloc/internal/arena_externs.h
@@ -106,7 +106,7 @@ unsigned arena_nthreads_get(arena_t *arena, bool internal);
 void arena_nthreads_inc(arena_t *arena, bool internal);
 void arena_nthreads_dec(arena_t *arena, bool internal);
 arena_t *arena_new(tsdn_t *tsdn, unsigned ind, const arena_config_t *config);
-bool arena_init_huge(arena_t *a0);
+bool arena_init_huge(tsdn_t *tsdn, arena_t *a0);
 arena_t *arena_choose_huge(tsd_t *tsd);
 bin_t *arena_bin_choose(tsdn_t *tsdn, arena_t *arena, szind_t binind,
     unsigned *binshard);
diff --git a/src/arena.c b/src/arena.c
index 84d4e14c..0a0c97ef 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -1904,7 +1904,7 @@ arena_choose_huge(tsd_t *tsd) {
 }
 
 bool
-arena_init_huge(arena_t *a0) {
+arena_init_huge(tsdn_t *tsdn, arena_t *a0) {
 	bool huge_enabled;
 	assert(huge_arena_ind == 0);
 
@@ -1922,13 +1922,18 @@ arena_init_huge(arena_t *a0) {
 		/* a0 init happened before malloc_conf_init. */
 		atomic_store_zu(&a0->pa_shard.pac.oversize_threshold,
 		    oversize_threshold, ATOMIC_RELAXED);
-		/* Initialize huge arena THP settings for PAC. */
+		/* Initialize huge_arena_pac_thp fields. */
+		base_t *b0 = a0->base;
+		/* Make sure that b0 thp auto-switch won't happen concurrently here. */
+		malloc_mutex_lock(tsdn, &b0->mtx);
 		(&huge_arena_pac_thp)->thp_madvise = opt_huge_arena_pac_thp &&
 		    metadata_thp_enabled() && (opt_thp == thp_mode_default) &&
 		    (init_system_thp_mode == thp_mode_default);
+		(&huge_arena_pac_thp)->auto_thp_switched = b0->auto_thp_switched;
 		malloc_mutex_init(&(&huge_arena_pac_thp)->lock, "pac_thp",
 		    WITNESS_RANK_LEAF, malloc_mutex_rank_exclusive);
 		edata_list_active_init(&(&huge_arena_pac_thp)->thp_lazy_list);
+		malloc_mutex_unlock(tsdn, &b0->mtx);
 		huge_enabled = true;
 	}
 
diff --git a/src/base.c b/src/base.c
index 13367697..52f3d1d3 100644
--- a/src/base.c
+++ b/src/base.c
@@ -153,17 +153,19 @@ base_get_num_blocks(base_t *base, bool with_new_block) {
 static void
 huge_arena_auto_thp_switch(tsdn_t *tsdn, pac_thp_t *pac_thp) {
 	assert(opt_huge_arena_pac_thp);
-	assert(!pac_thp->auto_thp_switched);
-
-	arena_t *huge_arena;
-	if (huge_arena_ind == 0 || (huge_arena = arena_get(tsdn, huge_arena_ind,
-	    false)) == NULL) {
-		/* Huge arena hasn't been init yet, simply turn the switch on. */
-		pac_thp->auto_thp_switched = true;
+#ifdef JEMALLOC_JET
+	if (pac_thp->auto_thp_switched) {
 		return;
 	}
+#else
+	/*
+	 * The switch should be turned on only once when the b0 auto thp switch is
+	 * turned on, unless it's a unit test where b0 gets deleted and then
+	 * recreated.
+	 */
+	assert(!pac_thp->auto_thp_switched);
+#endif
 
-	assert(huge_arena != NULL);
 	edata_list_active_t *pending_list;
 	malloc_mutex_lock(tsdn, &pac_thp->lock);
 	pending_list = &pac_thp->thp_lazy_list;
@@ -221,9 +223,19 @@ base_auto_thp_switch(tsdn_t *tsdn, base_t *base) {
 
 	/* Handle the THP auto switch for the huge arena. */
 	if (!huge_arena_pac_thp.thp_madvise || base_ind_get(base) != 0) {
-		/* Only b0 metadata auto thp switch do the trigger. */
+		/*
+		 * The huge arena THP auto-switch is triggered only by b0 switch,
+		 * provided that the huge arena is initialized. If b0 switch is enabled
+		 * before huge arena is ready, the huge arena switch will be enabled
+		 * during huge_arena_pac_thp initialization.
+		 */
 		return;
 	}
+	/*
+	 * thp_madvise above is by default false and set in arena_init_huge() with
+	 * b0 mtx held. So if we reach here, it means the entire huge_arena_pac_thp
+	 * is initialized and we can safely switch the THP.
+	 */
 	malloc_mutex_unlock(tsdn, &base->mtx);
 	huge_arena_auto_thp_switch(tsdn, &huge_arena_pac_thp);
 	malloc_mutex_lock(tsdn, &base->mtx);
diff --git a/src/jemalloc.c b/src/jemalloc.c
index 4939d954..d7b46d6c 100644
--- a/src/jemalloc.c
+++ b/src/jemalloc.c
@@ -2097,7 +2097,7 @@ percpu_arena_as_initialized(percpu_arena_mode_t mode) {
 }
 
 static bool
-malloc_init_narenas(void) {
+malloc_init_narenas(tsdn_t *tsdn) {
 	assert(ncpus > 0);
 
 	if (opt_percpu_arena != percpu_arena_disabled) {
@@ -2164,7 +2164,7 @@ malloc_init_narenas(void) {
 		    narenas_auto);
 	}
 	narenas_total_set(narenas_auto);
-	if (arena_init_huge(a0)) {
+	if (arena_init_huge(tsdn, a0)) {
 		narenas_total_inc();
 	}
 	manual_arena_base = narenas_total_get();
@@ -2248,7 +2248,7 @@ malloc_init_hard(void) {
 	/* Set reentrancy level to 1 during init. */
 	pre_reentrancy(tsd, NULL);
 	/* Initialize narenas before prof_boot2 (for allocation). */
-	if (malloc_init_narenas()
+	if (malloc_init_narenas(tsd_tsdn(tsd))
 	    || background_thread_boot1(tsd_tsdn(tsd), b0get())) {
 		UNLOCK_RETURN(tsd_tsdn(tsd), true, true)
 	}