From: Martin Pieuchot <mpi@grenadille.net>
Subject: uvm_fault_upper_lookup: delay pmap_extract()
To: tech@openbsd.org
Cc: tb@openbsd.org
Date: Thu, 28 Nov 2024 10:01:15 +0100

uvm_fault_upper_lookup() enters existing neighbor pages found in the
amap.

Diff below reorder the checks in the loop to avoid calling the expensive
pmap_extract(9) when there is no corresponding anon or if the page is
PG_RELEASED|PG_BUSY and won't be entered anyway.

While here call pmap_update() only if, at least, a page has been entered.

ok?

Index: uvm/uvm_fault.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_fault.c,v
diff -u -p -r1.146 uvm_fault.c
--- uvm/uvm_fault.c	27 Nov 2024 10:58:07 -0000	1.146
+++ uvm/uvm_fault.c	28 Nov 2024 08:35:10 -0000
@@ -841,10 +841,11 @@ uvm_fault_upper_lookup(struct uvm_faulti
 {
 	struct vm_amap *amap = ufi->entry->aref.ar_amap;
 	struct vm_anon *anon;
+	struct vm_page *pg;
 	boolean_t shadowed;
 	vaddr_t currva;
 	paddr_t pa;
-	int lcv;
+	int lcv, entered = 0;
 
 	/* locked: maps(read), amap(if there) */
 	KASSERT(amap == NULL ||
@@ -859,16 +860,6 @@ uvm_fault_upper_lookup(struct uvm_faulti
 	shadowed = FALSE;
 	for (lcv = 0; lcv < flt->npages; lcv++, currva += PAGE_SIZE) {
 		/*
-		 * dont play with VAs that are already mapped
-		 * except for center)
-		 */
-		if (lcv != flt->centeridx &&
-		    pmap_extract(ufi->orig_map->pmap, currva, &pa)) {
-			pages[lcv] = PGO_DONTCARE;
-			continue;
-		}
-
-		/*
 		 * unmapped or center page.   check if any anon at this level.
 		 */
 		if (amap == NULL || anons[lcv] == NULL) {
@@ -884,12 +875,20 @@ uvm_fault_upper_lookup(struct uvm_faulti
 			shadowed = TRUE;
 			continue;
 		}
+
 		anon = anons[lcv];
+		pg = anon->an_page;
+
 		KASSERT(anon->an_lock == amap->am_lock);
-		if (anon->an_page &&
-		    (anon->an_page->pg_flags & (PG_RELEASED|PG_BUSY)) == 0) {
+
+		/*
+		 * ignore busy pages.
+		 * don't play with VAs that are already mapped.
+		 */
+		if (pg && (pg->pg_flags & (PG_RELEASED|PG_BUSY)) == 0 &&
+		    !pmap_extract(ufi->orig_map->pmap, currva, &pa)) {
 			uvm_lock_pageq();
-			uvm_pageactivate(anon->an_page);	/* reactivate */
+			uvm_pageactivate(pg);	/* reactivate */
 			uvm_unlock_pageq();
 			counters_inc(uvmexp_counters, flt_namap);
 
@@ -902,13 +901,14 @@ uvm_fault_upper_lookup(struct uvm_faulti
 			 * that we enter these right now.
 			 */
 			(void) pmap_enter(ufi->orig_map->pmap, currva,
-			    VM_PAGE_TO_PHYS(anon->an_page) | flt->pa_flags,
+			    VM_PAGE_TO_PHYS(pg) | flt->pa_flags,
 			    (anon->an_ref > 1) ?
 			    (flt->enter_prot & ~PROT_WRITE) : flt->enter_prot,
 			    PMAP_CANFAIL);
+			entered++;
 		}
 	}
-	if (flt->npages > 1)
+	if (entered > 0)
 		pmap_update(ufi->orig_map->pmap);
 
 	return shadowed;