From: Martin Pieuchot <mpi@grenadille.net>
Subject: fault ahead & page wiring
To: tech@openbsd.org
Date: Mon, 25 Nov 2024 15:15:48 +0100

When a wired fault occurs, uvm_fault_check() disables looking & faulting
neighbor pages.  This is done line 702 by setting `narrow' to TRUE.

When a fault is "narrow" the kernel only looks at a single page and there
is no "fault-ahead".

So instead of syncing with NetBSD and using `flt->wired' coherently for
the pmap_enter(9) below, assert that such condition is impossible.

ok?

Index: uvm/uvm_fault.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_fault.c,v
diff -u -p -r1.144 uvm_fault.c
--- uvm/uvm_fault.c	25 Nov 2024 13:46:55 -0000	1.144
+++ uvm/uvm_fault.c	25 Nov 2024 14:01:07 -0000
@@ -888,6 +888,9 @@ uvm_fault_upper_lookup(struct uvm_faulti
 			uvm_unlock_pageq();
 			counters_inc(uvmexp_counters, flt_namap);
 
+			/* No fault-ahead when wired. */
+			KASSERT(flt->wired == 0);
+
 			/*
 			 * Since this isn't the page that's actually faulting,
 			 * ignore pmap_enter() failures; it's not critical
@@ -897,8 +900,7 @@ uvm_fault_upper_lookup(struct uvm_faulti
 			    VM_PAGE_TO_PHYS(anon->an_page) | flt->pa_flags,
 			    (anon->an_ref > 1) ?
 			    (flt->enter_prot & ~PROT_WRITE) : flt->enter_prot,
-			    PMAP_CANFAIL |
-			     (VM_MAPENT_ISWIRED(ufi->entry) ? PMAP_WIRED : 0));
+			    PMAP_CANFAIL);
 		}
 	}
 	if (flt->npages > 1)
@@ -1173,6 +1175,9 @@ uvm_fault_lower_lookup(
 		uvm_unlock_pageq();
 		counters_inc(uvmexp_counters, flt_nomap);
 
+		/* No fault-ahead when wired. */
+		KASSERT(flt->wired == 0);
+
 		/*
 		 * Since this page isn't the page that's
 		 * actually faulting, ignore pmap_enter()
@@ -1181,9 +1186,7 @@ uvm_fault_lower_lookup(
 		 */
 		(void) pmap_enter(ufi->orig_map->pmap, currva,
 		    VM_PAGE_TO_PHYS(pages[lcv]) | flt->pa_flags,
-		    flt->enter_prot & MASK(ufi->entry),
-		    PMAP_CANFAIL |
-		     (flt->wired ? PMAP_WIRED : 0));
+		    flt->enter_prot & MASK(ufi->entry), PMAP_CANFAIL);
 
 		/*
 		 * NOTE: page can't be PG_WANTED because