diff --git a/include/jemalloc/internal/edata.h b/include/jemalloc/internal/edata.h
index c71209e5..e75866ba 100644
--- a/include/jemalloc/internal/edata.h
+++ b/include/jemalloc/internal/edata.h
@@ -23,7 +23,7 @@ typedef enum extent_state_e extent_state_t;
 
 enum extent_head_state_e {
 	EXTENT_NOT_HEAD,
-	EXTENT_IS_HEAD   /* Only relevant for Windows && opt.retain. */
+	EXTENT_IS_HEAD   /* See comments in ehooks_default_merge_impl(). */
 };
 typedef enum extent_head_state_e extent_head_state_t;
 
diff --git a/include/jemalloc/internal/rtree.h b/include/jemalloc/internal/rtree.h
index 83dfdc81..3b7972e4 100644
--- a/include/jemalloc/internal/rtree.h
+++ b/include/jemalloc/internal/rtree.h
@@ -46,6 +46,7 @@ struct rtree_node_elm_s {
 typedef struct rtree_metadata_s rtree_metadata_t;
 struct rtree_metadata_s {
 	szind_t szind;
+	bool is_head; /* Mirrors edata->is_head. */
 	bool slab;
 };
 
@@ -65,9 +66,10 @@ struct rtree_leaf_elm_s {
 	 *
 	 * x: index
 	 * e: edata
+	 * h: is_head
 	 * b: slab
 	 *
-	 *   00000000 xxxxxxxx eeeeeeee [...] eeeeeeee eeee000b
+	 *   00000000 xxxxxxxx eeeeeeee [...] eeeeeeee eeee00hb
 	 */
 	atomic_p_t	le_bits;
 #else
@@ -184,12 +186,16 @@ rtree_leaf_elm_bits_encode(rtree_contents_t contents) {
 	    & (((uintptr_t)1 << LG_VADDR) - 1);
 	uintptr_t szind_bits = (uintptr_t)contents.metadata.szind << LG_VADDR;
 	/*
-	 * Slab shares the low bit of edata; we know edata is on an even address
-	 * (in fact, it's 128 bytes on 64-bit systems; we can enforce this
-	 * alignment if we want to steal 6 extra rtree leaf bits someday.
+	 * Metadata shares the low bits of edata. edata is CACHELINE aligned (in
+	 * fact, it's 128 bytes on 64-bit systems); we can enforce this
+	 * alignment if we want to steal the extra rtree leaf bits someday.
 	 */
 	uintptr_t slab_bits = (uintptr_t)contents.metadata.slab;
-	return szind_bits | edata_bits | slab_bits;
+	uintptr_t is_head_bits = (uintptr_t)contents.metadata.is_head << 1;
+	uintptr_t metadata_bits = szind_bits | is_head_bits | slab_bits;
+	assert((edata_bits & metadata_bits) == 0);
+
+	return edata_bits | metadata_bits;
 }
 
 JEMALLOC_ALWAYS_INLINE rtree_contents_t
@@ -198,20 +204,23 @@ rtree_leaf_elm_bits_decode(uintptr_t bits) {
 	/* Do the easy things first. */
 	contents.metadata.szind = bits >> LG_VADDR;
 	contents.metadata.slab = (bool)(bits & 1);
+	contents.metadata.is_head = (bool)(bits & (1 << 1));
+
+	uintptr_t metadata_mask = ~((uintptr_t)((1 << 2) - 1));
 #    ifdef __aarch64__
 	/*
 	 * aarch64 doesn't sign extend the highest virtual address bit to set
 	 * the higher ones.  Instead, the high bits get zeroed.
 	 */
 	uintptr_t high_bit_mask = ((uintptr_t)1 << LG_VADDR) - 1;
-	/* Mask off the slab bit. */
-	uintptr_t low_bit_mask = ~(uintptr_t)1;
+	/* Mask off metadata. */
+	uintptr_t low_bit_mask = metadata_mask;
 	uintptr_t mask = high_bit_mask & low_bit_mask;
 	contents.edata = (edata_t *)(bits & mask);
 #    else
-	/* Restore sign-extended high bits, mask slab bit. */
+	/* Restore sign-extended high bits, mask metadata bits. */
 	contents.edata = (edata_t *)((uintptr_t)((intptr_t)(bits << RTREE_NHIB)
-	    >> RTREE_NHIB) & ~((uintptr_t)0x1));
+	    >> RTREE_NHIB) & metadata_mask);
 #    endif
 	return contents;
 }
@@ -230,7 +239,8 @@ rtree_leaf_elm_read(tsdn_t *tsdn, rtree_t *rtree, rtree_leaf_elm_t *elm,
 	unsigned metadata_bits = atomic_load_u(&elm->le_metadata, dependent
 	    ? ATOMIC_RELAXED : ATOMIC_ACQUIRE);
 	contents.metadata.slab = (bool)(metadata_bits & 1);
-	contents.metadata.szind = (metadata_bits >> 1);
+	contents.metadata.is_head = (bool)(metadata_bits & (1 << 1));
+	contents.metadata.szind = (metadata_bits >> 2);
 
 	contents.edata = (edata_t *)atomic_load_p(&elm->le_edata, dependent
 	    ? ATOMIC_RELAXED : ATOMIC_ACQUIRE);
@@ -247,7 +257,8 @@ rtree_leaf_elm_write(tsdn_t *tsdn, rtree_t *rtree,
 	atomic_store_p(&elm->le_bits, (void *)bits, ATOMIC_RELEASE);
 #else
 	unsigned metadata_bits = ((unsigned)contents.metadata.slab
-	    | ((unsigned)contents.metadata.szind << 1));
+	    | ((unsigned)contents.metadata.is_head << 1)
+	    | ((unsigned)contents.metadata.szind << 2));
 	atomic_store_u(&elm->le_metadata, metadata_bits, ATOMIC_RELEASE);
 	/*
 	 * Write edata last, since the element is atomically considered valid
@@ -418,6 +429,7 @@ rtree_clear(tsdn_t *tsdn, rtree_t *rtree, rtree_ctx_t *rtree_ctx,
 	contents.edata = NULL;
 	contents.metadata.szind = SC_NSIZES;
 	contents.metadata.slab = false;
+	contents.metadata.is_head = false;
 	rtree_leaf_elm_write(tsdn, rtree, elm, contents);
 }
 
diff --git a/src/ehooks.c b/src/ehooks.c
index f2525e12..e1815ee8 100644
--- a/src/ehooks.c
+++ b/src/ehooks.c
@@ -227,10 +227,14 @@ bool
 ehooks_default_merge(extent_hooks_t *extent_hooks, void *addr_a, size_t size_a,
     void *addr_b, size_t size_b, bool committed, unsigned arena_ind) {
 	tsdn_t *tsdn = tsdn_fetch();
+
 	edata_t *a = emap_edata_lookup(tsdn, &arena_emap_global, addr_a);
 	bool head_a = edata_is_head_get(a);
 	edata_t *b = emap_edata_lookup(tsdn, &arena_emap_global, addr_b);
 	bool head_b = edata_is_head_get(b);
+	emap_assert_mapped(tsdn, &arena_emap_global, a);
+	emap_assert_mapped(tsdn, &arena_emap_global, b);
+
 	return ehooks_default_merge_impl(tsdn, addr_a, head_a, addr_b, head_b);
 }
 
diff --git a/src/emap.c b/src/emap.c
index 537f5884..62abf4d8 100644
--- a/src/emap.c
+++ b/src/emap.c
@@ -141,6 +141,8 @@ emap_rtree_write_acquired(tsdn_t *tsdn, emap_t *emap, rtree_leaf_elm_t *elm_a,
 	contents.edata = edata;
 	contents.metadata.szind = szind;
 	contents.metadata.slab = slab;
+	contents.metadata.is_head = (edata == NULL) ? false :
+	    edata_is_head_get(edata);
 	rtree_leaf_elm_write(tsdn, &emap->rtree, elm_a, contents);
 	if (elm_b != NULL) {
 		rtree_leaf_elm_write(tsdn, &emap->rtree, elm_b, contents);
@@ -169,12 +171,14 @@ emap_register_interior(tsdn_t *tsdn, emap_t *emap, edata_t *edata,
 
 	assert(edata_slab_get(edata));
 
+	rtree_contents_t contents;
+	contents.edata = edata;
+	contents.metadata.szind = szind;
+	contents.metadata.slab = true;
+	contents.metadata.is_head = false; /* Not allowed to access. */
+
 	/* Register interior. */
 	for (size_t i = 1; i < (edata_size_get(edata) >> LG_PAGE) - 1; i++) {
-		rtree_contents_t contents;
-		contents.edata = edata;
-		contents.metadata.szind = szind;
-		contents.metadata.slab = true;
 		rtree_write(tsdn, &emap->rtree, rtree_ctx,
 		    (uintptr_t)edata_base_get(edata) + (uintptr_t)(i <<
 		    LG_PAGE), contents);
@@ -214,6 +218,8 @@ emap_remap(tsdn_t *tsdn, emap_t *emap, edata_t *edata, szind_t szind,
 		contents.edata = edata;
 		contents.metadata.szind = szind;
 		contents.metadata.slab = slab;
+		contents.metadata.is_head = edata_is_head_get(edata);
+
 		rtree_write(tsdn, &emap->rtree, rtree_ctx,
 		    (uintptr_t)edata_addr_get(edata), contents);
 		/*
@@ -297,6 +303,7 @@ emap_merge_commit(tsdn_t *tsdn, emap_t *emap, emap_prepare_t *prepare,
 	clear_contents.edata = NULL;
 	clear_contents.metadata.szind = SC_NSIZES;
 	clear_contents.metadata.slab = false;
+	clear_contents.metadata.is_head = false;
 
 	if (prepare->lead_elm_b != NULL) {
 		rtree_leaf_elm_write(tsdn, &emap->rtree,
@@ -320,8 +327,10 @@ void
 emap_do_assert_mapped(tsdn_t *tsdn, emap_t *emap, edata_t *edata) {
 	EMAP_DECLARE_RTREE_CTX;
 
-	assert(rtree_read(tsdn, &emap->rtree, rtree_ctx,
-	    (uintptr_t)edata_base_get(edata)).edata == edata);
+	rtree_contents_t contents = rtree_read(tsdn, &emap->rtree, rtree_ctx,
+	    (uintptr_t)edata_base_get(edata));
+	assert(contents.edata == edata);
+	assert(contents.metadata.is_head == edata_is_head_get(edata));
 }
 
 void
diff --git a/src/extent.c b/src/extent.c
index 51711efc..a541e7bb 100644
--- a/src/extent.c
+++ b/src/extent.c
@@ -1254,6 +1254,8 @@ extent_merge_impl(tsdn_t *tsdn, pac_t *pac, ehooks_t *ehooks, edata_t *a,
 
 	assert(edata_arena_ind_get(a) == edata_arena_ind_get(b));
 	assert(edata_arena_ind_get(a) == ehooks_ind_get(ehooks));
+	emap_assert_mapped(tsdn, pac->emap, a);
+	emap_assert_mapped(tsdn, pac->emap, b);
 
 	bool err = ehooks_merge(tsdn, ehooks, edata_base_get(a),
 	    edata_size_get(a), edata_is_head_get(a), edata_base_get(b),
diff --git a/test/unit/rtree.c b/test/unit/rtree.c
index 775bc190..a547f188 100644
--- a/test/unit/rtree.c
+++ b/test/unit/rtree.c
@@ -55,6 +55,7 @@ TEST_BEGIN(test_rtree_extrema) {
 	contents_a.edata = &edata_a;
 	contents_a.metadata.szind = edata_szind_get(&edata_a);
 	contents_a.metadata.slab = edata_slab_get(&edata_a);
+	contents_a.metadata.is_head = edata_is_head_get(&edata_a);
 	expect_false(rtree_write(tsdn, rtree, &rtree_ctx, PAGE, contents_a),
 	    "Unexpected rtree_write() failure");
 	expect_false(rtree_write(tsdn, rtree, &rtree_ctx, PAGE, contents_a),
@@ -63,20 +64,23 @@ TEST_BEGIN(test_rtree_extrema) {
 	    PAGE);
 	expect_true(contents_a.edata == read_contents_a.edata
 	    && contents_a.metadata.szind == read_contents_a.metadata.szind
-	    && contents_a.metadata.slab == read_contents_a.metadata.slab,
+	    && contents_a.metadata.slab == read_contents_a.metadata.slab
+	    && contents_a.metadata.is_head == read_contents_a.metadata.is_head,
 	    "rtree_read() should return previously set value");
 
 	rtree_contents_t contents_b;
 	contents_b.edata = &edata_b;
 	contents_b.metadata.szind = edata_szind_get_maybe_invalid(&edata_b);
 	contents_b.metadata.slab = edata_slab_get(&edata_b);
+	contents_b.metadata.is_head = edata_is_head_get(&edata_b);
 	expect_false(rtree_write(tsdn, rtree, &rtree_ctx, ~((uintptr_t)0),
 	    contents_b), "Unexpected rtree_write() failure");
 	rtree_contents_t read_contents_b = rtree_read(tsdn, rtree, &rtree_ctx,
 	    ~((uintptr_t)0));
 	assert_true(contents_b.edata == read_contents_b.edata
 	    && contents_b.metadata.szind == read_contents_b.metadata.szind
-	    && contents_b.metadata.slab == read_contents_b.metadata.slab,
+	    && contents_b.metadata.slab == read_contents_b.metadata.slab
+	    && contents_b.metadata.is_head == read_contents_b.metadata.is_head,
 	    "rtree_read() should return previously set value");
 
 	base_delete(tsdn, base);
@@ -106,6 +110,7 @@ TEST_BEGIN(test_rtree_bits) {
 		contents.edata = &edata;
 		contents.metadata.szind = SC_NSIZES;
 		contents.metadata.slab = false;
+		contents.metadata.is_head = false;
 
 		expect_false(rtree_write(tsdn, rtree, &rtree_ctx, keys[i],
 		    contents), "Unexpected rtree_write() failure");
@@ -158,6 +163,7 @@ TEST_BEGIN(test_rtree_random) {
 		contents.edata = &edata;
 		contents.metadata.szind = SC_NSIZES;
 		contents.metadata.slab = false;
+		contents.metadata.is_head = false;
 		rtree_leaf_elm_write(tsdn, rtree, elm, contents);
 		expect_ptr_eq(rtree_read(tsdn, rtree, &rtree_ctx,
 		    keys[i]).edata, &edata,