As reported last month by a couple [0] of people [1] it is possible for
a pool_get(9) call with PR_NOWAIT to end up sleeping:

	panic() at assertwaitok+0xb8
	assertwaitok() at pool_get+0x34
	pool_get() at uvm_mapent_alloc+0x20c
	uvm_mapent_alloc() at uvm_map_clip_start+0x80
	uvm_map_clip_start() at uvm_unmap_remove+0x248
	uvm_unmap_remove() at uvm_unmap+0x64
	uvm_unmap() at km_free+0x50
	km_free() at pool_p_alloc+0x1f4
	pool_p_alloc() at pool_do_get+0x20c
	pool_do_get() at pool_get+0x8c
	pool_get() at pmap_vp_enter+0x17c
	pmap_vp_enter() at pmap_enter+0x1ac
	pmap_enter() at uvm_fault_lower+0x220
	uvm_fault_lower() at uvm_fault+0x158
	uvm_fault() at udata_abort+0x128

Diff below fixes that by allocating the pool page header *before* the
actual page.

This was enough to fix the issue, although I left a FIXME because the
pool_put() below might also end up returning a page to UVM which might
still want to sleep.  Returning the page should IMHO be delayed, at
least in the PR_NOWAIT case.

ok?

[0] https://marc.info/?l=openbsd-bugs&m=176831735127482&w=2
[1] https://marc.info/?l=openbsd-bugs&m=176839968920955&w=2

Index: kern/subr_pool.c
===================================================================
RCS file: /cvs/src/sys/kern/subr_pool.c,v
diff -u -p -r1.242 subr_pool.c
--- kern/subr_pool.c	1 Aug 2025 19:00:38 -0000	1.242
+++ kern/subr_pool.c	25 Jan 2026 13:04:25 -0000
@@ -923,19 +923,28 @@ pool_p_alloc(struct pool *pp, int flags,
 	pl_assert_unlocked(pp, &pp->pr_lock);
 	KASSERT(pp->pr_size >= sizeof(*pi));
 
+	if (!POOL_INPGHDR(pp)) {
+		ph = pool_get(&phpool, flags);
+		if (ph == NULL)
+			return (NULL);
+	}
+
 	addr = pool_allocator_alloc(pp, flags, slowdown);
-	if (addr == NULL)
+	if (addr == NULL) {
+		if (!POOL_INPGHDR(pp)) {
+			/*
+			 * FIXME: this can result in pool_p_free() calling
+			 * pool_allocator_free() then calling km_free(9).
+			 * km_free(9) might need to sleep and this is not
+			 * allowed with PR_NOWAIT.
+			 */
+			pool_put(&phpool, ph);
+		}
 		return (NULL);
+	}
 
 	if (POOL_INPGHDR(pp))
 		ph = (struct pool_page_header *)(addr + pp->pr_phoffset);
-	else {
-		ph = pool_get(&phpool, flags);
-		if (ph == NULL) {
-			pool_allocator_free(pp, addr);
-			return (NULL);
-		}
-	}
 
 	XSIMPLEQ_INIT(&ph->ph_items);
 	ph->ph_page = addr;