diff --git a/include/jemalloc/internal/hpdata.h b/include/jemalloc/internal/hpdata.h
index feca5f5e..30dd6721 100644
--- a/include/jemalloc/internal/hpdata.h
+++ b/include/jemalloc/internal/hpdata.h
@@ -363,8 +363,11 @@ struct hpdata_purge_state_s {
  * Once you begin purging, you have to follow through and call hpdata_purge_next
  * until you're done, and then end.  Allocating out of an hpdata undergoing
  * purging is not allowed.
+ *
+ * Returns the number of pages that will be purged.
  */
-void hpdata_purge_begin(hpdata_t *hpdata, hpdata_purge_state_t *purge_state);
+size_t hpdata_purge_begin(hpdata_t *hpdata, hpdata_purge_state_t *purge_state);
+
 /*
  * If there are more extents to purge, sets *r_purge_addr and *r_purge_size to
  * true, and returns true.  Otherwise, returns false to indicate that we're
diff --git a/src/hpdata.c b/src/hpdata.c
index bb4808aa..e11ba8d9 100644
--- a/src/hpdata.c
+++ b/src/hpdata.c
@@ -158,7 +158,7 @@ hpdata_unreserve(hpdata_t *hpdata, void *addr, size_t sz) {
 	hpdata_assert_consistent(hpdata);
 }
 
-void
+size_t
 hpdata_purge_begin(hpdata_t *hpdata, hpdata_purge_state_t *purge_state) {
 	hpdata_assert_consistent(hpdata);
 	/* See the comment in reserve. */
@@ -181,10 +181,13 @@ hpdata_purge_begin(hpdata_t *hpdata, hpdata_purge_state_t *purge_state) {
 	    hpdata->touched_pages, HUGEPAGE_PAGES);
 
 	/* We purge everything we can. */
-	assert(hpdata->h_ntouched - hpdata->h_nactive == fb_scount(
+	size_t to_purge = hpdata->h_ntouched - hpdata->h_nactive;
+	assert(to_purge == fb_scount(
 	    purge_state->to_purge, HUGEPAGE_PAGES, 0, HUGEPAGE_PAGES));
 
 	hpdata_assert_consistent(hpdata);
+
+	return to_purge;
 }
 
 bool
diff --git a/test/unit/hpdata.c b/test/unit/hpdata.c
index 2a702338..11bccc58 100644
--- a/test/unit/hpdata.c
+++ b/test/unit/hpdata.c
@@ -68,7 +68,8 @@ TEST_BEGIN(test_purge_simple) {
 	expect_zu_eq(hpdata_ntouched_get(&hpdata), HUGEPAGE_PAGES / 2, "");
 
 	hpdata_purge_state_t purge_state;
-	hpdata_purge_begin(&hpdata, &purge_state);
+	size_t to_purge = hpdata_purge_begin(&hpdata, &purge_state);
+	expect_zu_eq(HUGEPAGE_PAGES / 4, to_purge, "");
 
 	void *purge_addr;
 	size_t purge_size;
@@ -112,7 +113,8 @@ TEST_BEGIN(test_purge_intervening_dalloc) {
 	expect_zu_eq(hpdata_ntouched_get(&hpdata), 3 * HUGEPAGE_PAGES / 4, "");
 
 	hpdata_purge_state_t purge_state;
-	hpdata_purge_begin(&hpdata, &purge_state);
+	size_t to_purge = hpdata_purge_begin(&hpdata, &purge_state);
+	expect_zu_eq(HUGEPAGE_PAGES / 2, to_purge, "");
 
 	void *purge_addr;
 	size_t purge_size;