From: Martin Pieuchot <mpi@grenadille.net>
Subject: Re: fault ahead & page wiring
To: tech@openbsd.org
Date: Wed, 27 Nov 2024 11:04:26 +0100

On 25/11/24(Mon) 15:15, Martin Pieuchot wrote:
> 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.

I have been asked off-list why this KASSERT() couldn't be reached.  This
is because when the fault is "narrow", which is always the case for wired
faults, only a single page is mapped in the memory space.  Line 775 we
see:

	f (flt->narrow == FALSE) {
		[...]
        } else {
                /* narrow fault! */
                nback = nforw = 0;
                flt->startva = ufi->orig_rvaddr;
                flt->npages = 1;
                flt->centeridx = 0;
        }

`npages' being 1 and `centeridx' being 0 ensure that the first and only page 
found in the iterations below will be skipped.
d
While here use FALSE instead of 0 to be consistent with the rest of the
file.

ok?

Index: uvm/uvm_fault.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_fault.c,v
diff -u -p -r1.145 uvm_fault.c
--- uvm/uvm_fault.c	26 Nov 2024 10:10:28 -0000	1.145
+++ uvm/uvm_fault.c	27 Nov 2024 09:50:53 -0000
@@ -893,6 +893,9 @@ uvm_fault_upper_lookup(struct uvm_faulti
 			uvm_unlock_pageq();
 			counters_inc(uvmexp_counters, flt_namap);
 
+			/* No fault-ahead when wired. */
+			KASSERT(flt->wired == FALSE);
+
 			/*
 			 * Since this isn't the page that's actually faulting,
 			 * ignore pmap_enter() failures; it's not critical
@@ -902,8 +905,7 @@ 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)
@@ -1167,6 +1169,9 @@ uvm_fault_lower_lookup(
 		uvm_unlock_pageq();
 		counters_inc(uvmexp_counters, flt_nomap);
 
+		/* No fault-ahead when wired. */
+		KASSERT(flt->wired == FALSE);
+
 		/*
 		 * Since this page isn't the page that's
 		 * actually faulting, ignore pmap_enter()
@@ -1175,9 +1180,7 @@ 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