It is currently impossible to use km_alloc(9) to get a managed mapping
backed with physical pages allocated up front.  I guess this is why
uvm_km_kmemalloc_pla() is still used to allocate the UAREA.

To fix that, I suggest the diff below which turns the "kp_pageable" flag
of km_alloc(9) into a "kp_managed".  This new flag no longer implies
"kp_nomem" so I updated the description and fixed the remaining XXX to
preserve existing behavior of the `kp_pageable' global.

With this change I believe we could also get rid of uvm_km_zalloc() used
in the i386 pmap.

Comments?  Oks?

Index: sys/uvm/uvm_extern.h
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_extern.h,v
diff -u -p -r1.177 uvm_extern.h
--- sys/uvm/uvm_extern.h	24 Aug 2024 10:46:43 -0000	1.177
+++ sys/uvm/uvm_extern.h	2 Oct 2024 14:51:32 -0000
@@ -328,7 +328,7 @@ struct kmem_va_mode {
  *  kp_nomem - don't allocate any backing pages.
  *  kp_maxseg - maximal amount of contiguous segments.
  *  kp_zero - zero the returned memory.
- *  kp_pageable - allocate pageable memory.
+ *  kp_managed - map memory paged from `kp_object'.
  */
 struct kmem_pa_mode {
 	struct uvm_constraint_range *kp_constraint;
@@ -338,7 +338,7 @@ struct kmem_pa_mode {
 	int kp_maxseg;
 	char kp_nomem;
 	char kp_zero;
-	char kp_pageable;
+	char kp_managed;
 };
 
 /*
Index: sys/uvm/uvm_glue.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_glue.c,v
diff -u -p -r1.84 uvm_glue.c
--- sys/uvm/uvm_glue.c	10 Sep 2022 20:35:29 -0000	1.84
+++ sys/uvm/uvm_glue.c	2 Oct 2024 14:39:14 -0000
@@ -259,20 +259,26 @@ uvm_vsunlock_device(struct proc *p, void
 	uvm_km_free(kernel_map, kva, sz);
 }
 
+const struct kmem_va_mode kv_uarea = {
+	.kv_map = &kernel_map,
+	.kv_align = USPACE_ALIGN
+};
+
+const struct kmem_pa_mode kp_uarea = {
+	.kp_constraint = &no_constraint,
+	.kp_object = &uvm.kernel_object,
+	.kp_zero = 1,
+	.kp_managed = 1
+};
+
 /*
  * uvm_uarea_alloc: allocate the u-area for a new thread
  */
 vaddr_t
 uvm_uarea_alloc(void)
 {
-	vaddr_t uaddr;
-
-	uaddr = uvm_km_kmemalloc_pla(kernel_map, uvm.kernel_object, USPACE,
-	    USPACE_ALIGN, UVM_KMF_ZERO,
-	    no_constraint.ucr_low, no_constraint.ucr_high,
-	    0, 0, USPACE/PAGE_SIZE);
 
-	return (uaddr);
+	return (vaddr_t)km_alloc(USPACE, &kv_uarea, &kp_uarea, &kd_waitok);
 }
 
 /*
@@ -284,7 +290,7 @@ uvm_uarea_alloc(void)
 void
 uvm_uarea_free(struct proc *p)
 {
-	uvm_km_free(kernel_map, (vaddr_t)p->p_addr, USPACE);
+	km_free(p->p_addr, USPACE, &kv_uarea, &kp_uarea);
 	p->p_addr = NULL;
 }
 
Index: sys/uvm/uvm_km.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_km.c,v
diff -u -p -r1.154 uvm_km.c
--- sys/uvm/uvm_km.c	24 Aug 2024 10:46:43 -0000	1.154
+++ sys/uvm/uvm_km.c	2 Oct 2024 14:49:00 -0000
@@ -743,7 +743,7 @@ km_alloc(size_t sz, const struct kmem_va
 
 	TAILQ_INIT(&pgl);
 
-	if (kp->kp_nomem || kp->kp_pageable)
+	if (kp->kp_nomem)
 		goto alloc_va;
 
 	pla_flags = kd->kd_waitok ? UVM_PLA_WAITOK : UVM_PLA_NOWAIT;
@@ -753,7 +753,8 @@ km_alloc(size_t sz, const struct kmem_va
 
 	pla_align = kp->kp_align;
 #ifdef __HAVE_PMAP_DIRECT
-	if (pla_align < kv->kv_align)
+	if ((kv->kv_singlepage || kp->kp_maxseg == 1) &&
+	    (pla_align < kv->kv_align))
 		pla_align = kv->kv_align;
 #endif
 	pla_maxseg = kp->kp_maxseg;
@@ -762,7 +763,7 @@ km_alloc(size_t sz, const struct kmem_va
 
 	if (uvm_pglistalloc(sz, kp->kp_constraint->ucr_low,
 	    kp->kp_constraint->ucr_high, pla_align, kp->kp_boundary,
-	    &pgl, pla_maxseg, pla_flags)) {	
+	    &pgl, pla_maxseg, pla_flags)) {
 		return (NULL);
 	}
 
@@ -783,7 +784,7 @@ km_alloc(size_t sz, const struct kmem_va
 alloc_va:
 	prot = PROT_READ | PROT_WRITE;
 
-	if (kp->kp_pageable) {
+	if (kp->kp_managed) {
 		KASSERT(kp->kp_object);
 		KASSERT(!kv->kv_singlepage);
 	} else {
@@ -838,7 +839,7 @@ try_map:
 	}
 	sva = va;
 	TAILQ_FOREACH(pg, &pgl, pageq) {
-		if (kp->kp_pageable)
+		if (kp->kp_managed)
 			pmap_enter(pmap_kernel(), va, VM_PAGE_TO_PHYS(pg),
 			    prot, prot | PMAP_WIRED);
 		else
@@ -860,9 +861,6 @@ km_free(void *v, size_t sz, const struct
 	sva = (vaddr_t)v;
 	eva = sva + sz;
 
-	if (kp->kp_nomem)
-		goto free_va;
-
 #ifdef __HAVE_PMAP_DIRECT
 	if (kv->kv_singlepage || kp->kp_maxseg == 1) {
 		TAILQ_INIT(&pgl);
@@ -887,10 +885,10 @@ km_free(void *v, size_t sz, const struct
 	}
 #endif
 
-	if (kp->kp_pageable) {
+	if (kp->kp_managed) {
 		pmap_remove(pmap_kernel(), sva, eva);
 		pmap_update(pmap_kernel());
-	} else {
+	} else if (!kp->kp_nomem) {
 		TAILQ_INIT(&pgl);
 		for (va = sva; va < eva; va += PAGE_SIZE) {
 			paddr_t pa;
@@ -908,7 +906,7 @@ km_free(void *v, size_t sz, const struct
 		pmap_update(pmap_kernel());
 		uvm_pglistfree(&pgl);
 	}
-free_va:
+
 	uvm_unmap(*kv->kv_map, sva, eva);
 	if (kv->kv_wait)
 		wakeup(*kv->kv_map);
@@ -951,8 +949,8 @@ const struct kmem_pa_mode kp_zero = {
 
 const struct kmem_pa_mode kp_pageable = {
 	.kp_object = &uvm.kernel_object,
-	.kp_pageable = 1
-/* XXX - kp_nomem, maybe, but we'll need to fix km_free. */
+	.kp_managed = 1,
+	.kp_nomem = 1
 };
 
 const struct kmem_pa_mode kp_none = {
Index: share/man/man9/km_alloc.9
===================================================================
RCS file: /cvs/src/share/man/man9/km_alloc.9,v
diff -u -p -r1.9 km_alloc.9
--- share/man/man9/km_alloc.9	6 Dec 2019 19:15:16 -0000	1.9
+++ share/man/man9/km_alloc.9	2 Oct 2024 14:47:18 -0000
@@ -126,7 +126,7 @@ struct kmem_pa_mode {
 	int kp_nomem;
 	int kp_maxseg;
 	int kp_zero;
-	int kp_pageable;
+	int kp_managed;
 };
 .Ed
 .Bl -tag -width kp_constraint
@@ -141,15 +141,14 @@ Physical alignment of the first page in 
 Boundary that the physical addresses can't cross if the allocation is
 contiguous.
 .It kp_nomem
-A flag that specifies that the allocation should not be backed by physical
-pages.
+A flag that specifies that no physical page should be allocated up front.
 .It kp_maxseg
 Maximal amount of contiguous physical segments in the allocation.
 .It kp_zero
 A flag that specifies if the returned memory should be zeroed.
-.It kp_pageable
-A flag that specifies if the returned memory should be demand paged from the
-backing object instead of being allocated up front.
+.It kp_managed
+A flag that specifies if the returned memory should be mapped paged from the
+backing object.
 .El
 .Bd -literal
 struct kmem_dyn_mode {