diff --git a/sys/arm64/arm64/pmap.c b/sys/arm64/arm64/pmap.c
--- a/sys/arm64/arm64/pmap.c
+++ b/sys/arm64/arm64/pmap.c
@@ -1870,6 +1870,8 @@
 
 		l0index = ptepindex - (NUL2E + NUL1E);
 		l0 = &pmap->pm_l0[l0index];
+		KASSERT((pmap_load(l0) & ATTR_DESCR_VALID) == 0,
+		    ("%s: L0 entry %#lx is valid", __func__, pmap_load(l0)));
 		pmap_store(l0, VM_PAGE_TO_PHYS(m) | L0_TABLE);
 	} else if (ptepindex >= NUL2E) {
 		vm_pindex_t l0index, l1index;
@@ -1896,6 +1898,8 @@
 
 		l1 = (pd_entry_t *)PHYS_TO_DMAP(pmap_load(l0) & ~ATTR_MASK);
 		l1 = &l1[ptepindex & Ln_ADDR_MASK];
+		KASSERT((pmap_load(l1) & ATTR_DESCR_VALID) == 0,
+		    ("%s: L1 entry %#lx is valid", __func__, pmap_load(l1)));
 		pmap_store(l1, VM_PAGE_TO_PHYS(m) | L1_TABLE);
 	} else {
 		vm_pindex_t l0index, l1index;
@@ -1938,6 +1942,8 @@
 
 		l2 = (pd_entry_t *)PHYS_TO_DMAP(pmap_load(l1) & ~ATTR_MASK);
 		l2 = &l2[ptepindex & Ln_ADDR_MASK];
+		KASSERT((pmap_load(l2) & ATTR_DESCR_VALID) == 0,
+		    ("%s: L2 entry %#lx is valid", __func__, pmap_load(l2)));
 		pmap_store(l2, VM_PAGE_TO_PHYS(m) | L2_TABLE);
 	}
 
diff --git a/sys/i386/i386/pmap.c b/sys/i386/i386/pmap.c
--- a/sys/i386/i386/pmap.c
+++ b/sys/i386/i386/pmap.c
@@ -2157,8 +2157,11 @@
 	pmap->pm_stats.resident_count++;
 
 	ptepa = VM_PAGE_TO_PHYS(m);
+	KASSERT((pmap->pm_pdir[ptepindex] & PG_V) == 0,
+	    ("%s: page directory entry %#jx is valid",
+	    __func__, (uintmax_t)pmap->pm_pdir[ptepindex]));
 	pmap->pm_pdir[ptepindex] =
-		(pd_entry_t) (ptepa | PG_U | PG_RW | PG_V | PG_A | PG_M);
+	    (pd_entry_t)(ptepa | PG_U | PG_RW | PG_V | PG_A | PG_M);
 
 	return (m);
 }
diff --git a/sys/mips/mips/pmap.c b/sys/mips/mips/pmap.c
--- a/sys/mips/mips/pmap.c
+++ b/sys/mips/mips/pmap.c
@@ -1217,9 +1217,13 @@
 		}
 		/* Next level entry */
 		pde = (pd_entry_t *)*pdep;
+		KASSERT(pde[pdeindex] == 0,
+		    ("%s: PTE %p is valid", __func__, pde[pdeindex]));
 		pde[pdeindex] = (pd_entry_t)pageva;
 	}
 #else
+	KASSERT(pmap->pm_segtab[ptepindex] == 0,
+	    ("%s: PTE %p is valid", __func__, pmap->pm_segtab[ptepindex]));
 	pmap->pm_segtab[ptepindex] = (pd_entry_t)pageva;
 #endif
 	pmap->pm_stats.resident_count++;
diff --git a/sys/powerpc/aim/mmu_radix.c b/sys/powerpc/aim/mmu_radix.c
--- a/sys/powerpc/aim/mmu_radix.c
+++ b/sys/powerpc/aim/mmu_radix.c
@@ -4248,8 +4248,9 @@
 		/* Wire up a new PDPE page */
 		pml1index = ptepindex - (NUPDE + NUPDPE);
 		l1e = &pmap->pm_pml1[pml1index];
+		KASSERT((be64toh(*l1e) & PG_V) == 0,
+		    ("%s: L1 entry %#lx is valid", __func__, *l1e));
 		pde_store(l1e, VM_PAGE_TO_PHYS(m));
-
 	} else if (ptepindex >= NUPDE) {
 		vm_pindex_t pml1index;
 		vm_pindex_t pdpindex;
@@ -4278,8 +4279,9 @@
 
 		/* Now find the pdp page */
 		l2e = &l2e[pdpindex & RPTE_MASK];
+		KASSERT((be64toh(*l2e) & PG_V) == 0,
+		    ("%s: L2 entry %#lx is valid", __func__, *l2e));
 		pde_store(l2e, VM_PAGE_TO_PHYS(m));
-
 	} else {
 		vm_pindex_t pml1index;
 		vm_pindex_t pdpindex;
@@ -4324,6 +4326,8 @@
 
 		/* Now we know where the page directory page is */
 		l3e = &l3e[ptepindex & RPTE_MASK];
+		KASSERT((be64toh(*l3e) & PG_V) == 0,
+		    ("%s: L3 entry %#lx is valid", __func__, *l3e));
 		pde_store(l3e, VM_PAGE_TO_PHYS(m));
 	}
 
diff --git a/sys/riscv/riscv/pmap.c b/sys/riscv/riscv/pmap.c
--- a/sys/riscv/riscv/pmap.c
+++ b/sys/riscv/riscv/pmap.c
@@ -1285,6 +1285,8 @@
 
 		l1index = ptepindex - NUL1E;
 		l1 = &pmap->pm_l1[l1index];
+		KASSERT((pmap_load(l1) & PTE_V) == 0,
+		    ("%s: L1 entry %#lx is valid", __func__, pmap_load(l1)));
 
 		pn = (VM_PAGE_TO_PHYS(m) / PAGE_SIZE);
 		entry = (PTE_V);
@@ -1314,6 +1316,8 @@
 		phys = PTE_TO_PHYS(pmap_load(l1));
 		l2 = (pd_entry_t *)PHYS_TO_DMAP(phys);
 		l2 = &l2[ptepindex & Ln_ADDR_MASK];
+		KASSERT((pmap_load(l2) & PTE_V) == 0,
+		    ("%s: L2 entry %#lx is valid", __func__, pmap_load(l2)));
 
 		pn = (VM_PAGE_TO_PHYS(m) / PAGE_SIZE);
 		entry = (PTE_V);