diff --git a/sys/miscfs/genfs/genfs_vnops.c b/sys/miscfs/genfs/genfs_vnops.c
index 8278507050d9..512e5d6fc116 100644
--- a/sys/miscfs/genfs/genfs_vnops.c
+++ b/sys/miscfs/genfs/genfs_vnops.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: genfs_vnops.c,v 1.99 2005/07/16 03:54:08 yamt Exp $	*/
+/*	$NetBSD: genfs_vnops.c,v 1.100 2005/07/17 09:13:35 yamt Exp $	*/
 
 /*
  * Copyright (c) 1982, 1986, 1989, 1993
@@ -31,7 +31,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: genfs_vnops.c,v 1.99 2005/07/16 03:54:08 yamt Exp $");
+__KERNEL_RCSID(0, "$NetBSD: genfs_vnops.c,v 1.100 2005/07/17 09:13:35 yamt Exp $");
 
 #if defined(_KERNEL_OPT)
 #include "opt_nfsserver.h"
@@ -496,6 +496,7 @@ genfs_getpages(void *v)
 	boolean_t write = (ap->a_access_type & VM_PROT_WRITE) != 0;
 	boolean_t sawhole = FALSE;
 	boolean_t overwrite = (flags & PGO_OVERWRITE) != 0;
+	boolean_t blockalloc = write && (flags & PGO_NOBLOCKALLOC) == 0;
 	UVMHIST_FUNC("genfs_getpages"); UVMHIST_CALLED(ubchist);
 
 	UVMHIST_LOG(ubchist, "vp %p off 0x%x/%x count %d",
@@ -606,7 +607,7 @@ genfs_getpages(void *v)
 		struct vm_page *pg1 = pgs[ridx + i];
 
 		if ((pg1->flags & PG_FAKE) ||
-		    (write && (pg1->flags & PG_RDONLY))) {
+		    (blockalloc && (pg1->flags & PG_RDONLY))) {
 			break;
 		}
 	}
@@ -711,7 +712,7 @@ genfs_getpages(void *v)
 	 * now loop over the pages, reading as needed.
 	 */
 
-	if (write) {
+	if (blockalloc) {
 		lockmgr(&gp->g_glock, LK_EXCLUSIVE, NULL);
 	} else {
 		lockmgr(&gp->g_glock, LK_SHARED, NULL);
@@ -727,10 +728,13 @@ genfs_getpages(void *v)
 		 */
 
 		pidx = (offset - startoffset) >> PAGE_SHIFT;
-		while ((pgs[pidx]->flags & (PG_FAKE|PG_RDONLY)) == 0) {
+		while ((pgs[pidx]->flags & PG_FAKE) == 0) {
 			size_t b;
 
 			KASSERT((offset & (PAGE_SIZE - 1)) == 0);
+			if ((pgs[pidx]->flags & PG_RDONLY)) {
+				sawhole = TRUE;
+			}
 			b = MIN(PAGE_SIZE, bytes);
 			offset += b;
 			bytes -= b;
@@ -778,8 +782,8 @@ genfs_getpages(void *v)
 
 		/*
 		 * if this block isn't allocated, zero it instead of
-		 * reading it.  if this is a read access, mark the
-		 * pages we zeroed PG_RDONLY.
+		 * reading it.  unless we are going to allocate blocks,
+		 * mark the pages we zeroed PG_RDONLY.
 		 */
 
 		if (blkno < 0) {
@@ -795,7 +799,8 @@ genfs_getpages(void *v)
 			for (i = 0; i < holepages; i++) {
 				if (write) {
 					pgs[pidx + i]->flags &= ~PG_CLEAN;
-				} else {
+				}
+				if (!blockalloc) {
 					pgs[pidx + i]->flags |= PG_RDONLY;
 				}
 			}
@@ -881,18 +886,21 @@ loopdone:
 	 * the page is completely allocated while the pages are locked.
 	 */
 
-	if (!error && sawhole && write) {
-		for (i = 0; i < npages; i++) {
-			if (pgs[i] == NULL) {
-				continue;
-			}
-			pgs[i]->flags &= ~PG_CLEAN;
-			UVMHIST_LOG(ubchist, "mark dirty pg %p", pgs[i],0,0,0);
-		}
+	if (!error && sawhole && blockalloc) {
 		error = GOP_ALLOC(vp, startoffset, npages << PAGE_SHIFT, 0,
 		    cred);
 		UVMHIST_LOG(ubchist, "gop_alloc off 0x%x/0x%x -> %d",
 		    startoffset, npages << PAGE_SHIFT, error,0);
+		if (!error) {
+			for (i = 0; i < npages; i++) {
+				if (pgs[i] == NULL) {
+					continue;
+				}
+				pgs[i]->flags &= ~(PG_CLEAN|PG_RDONLY);
+				UVMHIST_LOG(ubchist, "mark dirty pg %p",
+				    pgs[i],0,0,0);
+			}
+		}
 	}
 	lockmgr(&gp->g_glock, LK_RELEASE, NULL);
 	simple_lock(&uobj->vmobjlock);
@@ -974,9 +982,7 @@ out:
 			pg->flags &= ~(PG_FAKE);
 			pmap_clear_modify(pgs[i]);
 		}
-		if (write) {
-			pg->flags &= ~(PG_RDONLY);
-		}
+		KASSERT(!write || !blockalloc || (pg->flags & PG_RDONLY) == 0);
 		if (i < ridx || i >= ridx + orignpages || async) {
 			UVMHIST_LOG(ubchist, "unbusy pg %p offset 0x%x",
 			    pg, pg->offset,0,0);
diff --git a/sys/ufs/ufs/ufs_inode.c b/sys/ufs/ufs/ufs_inode.c
index e70c71fc7ae4..7570266e14c7 100644
--- a/sys/ufs/ufs/ufs_inode.c
+++ b/sys/ufs/ufs/ufs_inode.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: ufs_inode.c,v 1.49 2005/07/10 01:08:52 thorpej Exp $	*/
+/*	$NetBSD: ufs_inode.c,v 1.50 2005/07/17 09:13:35 yamt Exp $	*/
 
 /*
  * Copyright (c) 1991, 1993
@@ -37,7 +37,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: ufs_inode.c,v 1.49 2005/07/10 01:08:52 thorpej Exp $");
+__KERNEL_RCSID(0, "$NetBSD: ufs_inode.c,v 1.50 2005/07/17 09:13:35 yamt Exp $");
 
 #if defined(_KERNEL_OPT)
 #include "opt_ffs.h"
@@ -235,7 +235,7 @@ ufs_balloc_range(struct vnode *vp, off_t off, off_t len, struct ucred *cred,
 	memset(pgs, 0, npages * sizeof(struct vm_page *));
 	simple_lock(&uobj->vmobjlock);
 	error = VOP_GETPAGES(vp, pagestart, pgs, &npages, 0,
-	    VM_PROT_READ, 0, PGO_SYNCIO|PGO_PASTEOF);
+	    VM_PROT_WRITE, 0, PGO_SYNCIO|PGO_PASTEOF|PGO_NOBLOCKALLOC);
 	if (error) {
 		return error;
 	}
@@ -273,9 +273,11 @@ ufs_balloc_range(struct vnode *vp, off_t off, off_t len, struct ucred *cred,
 
 	simple_lock(&uobj->vmobjlock);
 	for (i = 0; i < npages; i++) {
-		pgs[i]->flags &= ~PG_RDONLY;
 		if (error) {
 			pgs[i]->flags |= PG_RELEASED;
+		} else if (off <= pagestart + (i << PAGE_SHIFT) &&
+		    pagestart + ((i + 1) << PAGE_SHIFT) <= off + len) {
+			pgs[i]->flags &= ~PG_RDONLY;
 		}
 	}
 	if (error) {
diff --git a/sys/uvm/uvm_bio.c b/sys/uvm/uvm_bio.c
index 113678685546..32fcbb16fdf9 100644
--- a/sys/uvm/uvm_bio.c
+++ b/sys/uvm/uvm_bio.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: uvm_bio.c,v 1.39 2005/06/27 02:19:48 thorpej Exp $	*/
+/*	$NetBSD: uvm_bio.c,v 1.40 2005/07/17 09:13:35 yamt Exp $	*/
 
 /*
  * Copyright (c) 1998 Chuck Silvers.
@@ -34,7 +34,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: uvm_bio.c,v 1.39 2005/06/27 02:19:48 thorpej Exp $");
+__KERNEL_RCSID(0, "$NetBSD: uvm_bio.c,v 1.40 2005/07/17 09:13:35 yamt Exp $");
 
 #include "opt_uvmhist.h"
 
@@ -276,7 +276,7 @@ again:
 	    uobj, umap->offset + slot_offset, npages, 0);
 
 	error = (*uobj->pgops->pgo_get)(uobj, umap->offset + slot_offset, pgs,
-	    &npages, 0, access_type, 0, flags);
+	    &npages, 0, access_type, 0, flags | PGO_NOBLOCKALLOC);
 	UVMHIST_LOG(ubchist, "getpages error %d npages %d", error, npages, 0,
 	    0);
 
@@ -295,6 +295,8 @@ again:
 	simple_lock(&uobj->vmobjlock);
 	uvm_lock_pageq();
 	for (i = 0; va < eva; i++, va += PAGE_SIZE) {
+		boolean_t rdonly;
+		vm_prot_t mask;
 
 		/*
 		 * for virtually-indexed, virtually-tagged caches we should
@@ -340,11 +342,21 @@ again:
 					continue; /* will re-fault */
 			}
 		}
-		KASSERT(access_type == VM_PROT_READ ||
-		    (pg->flags & PG_RDONLY) == 0);
+
+		/*
+		 * note that a page whose backing store is partially allocated
+		 * is marked as PG_RDONLY.
+		 */
+
+		rdonly = (access_type & VM_PROT_WRITE) == 0 &&
+		    (pg->flags & PG_RDONLY) != 0;
+		KASSERT((pg->flags & PG_RDONLY) == 0 ||
+		    (access_type & VM_PROT_WRITE) == 0 ||
+		    pg->offset < umap->writeoff ||
+		    pg->offset + PAGE_SIZE > umap->writeoff + umap->writelen);
+		mask = rdonly ? ~VM_PROT_WRITE : VM_PROT_ALL;
 		pmap_enter(ufi->orig_map->pmap, va, VM_PAGE_TO_PHYS(pg),
-		    (pg->flags & PG_RDONLY) ? prot & ~VM_PROT_WRITE : prot,
-		    access_type);
+		    prot & mask, access_type & mask);
 		uvm_pageactivate(pg);
 		pg->flags &= ~(PG_BUSY);
 		UVM_PAGE_OWN(pg, NULL);
@@ -456,7 +468,8 @@ again:
 	if (flags & UBC_FAULTBUSY) {
 		int npages = (*lenp + PAGE_SIZE - 1) >> PAGE_SHIFT;
 		struct vm_page *pgs[npages];
-		int gpflags = PGO_SYNCIO|PGO_OVERWRITE|PGO_PASTEOF;
+		int gpflags =
+		    PGO_SYNCIO|PGO_OVERWRITE|PGO_PASTEOF|PGO_NOBLOCKALLOC;
 		int i;
 		KDASSERT(flags & UBC_WRITE);
 
diff --git a/sys/uvm/uvm_pager.h b/sys/uvm/uvm_pager.h
index 9967456702f0..442e54b6b123 100644
--- a/sys/uvm/uvm_pager.h
+++ b/sys/uvm/uvm_pager.h
@@ -1,4 +1,4 @@
-/*	$NetBSD: uvm_pager.h,v 1.28 2004/03/24 07:55:01 junyoung Exp $	*/
+/*	$NetBSD: uvm_pager.h,v 1.29 2005/07/17 09:13:35 yamt Exp $	*/
 
 /*
  *
@@ -160,6 +160,7 @@ struct uvm_pagerops {
 #define PGO_BUSYFAIL	0x080	/* fail if a page is busy [put] */
 #define PGO_OVERWRITE	0x200	/* pages will be overwritten before unlocked */
 #define PGO_PASTEOF	0x400	/* allow allocation of pages past EOF */
+#define PGO_NOBLOCKALLOC 0x800	/* backing block allocation is not needed */
 
 /* page we are not interested in getting */
 #define PGO_DONTCARE ((struct vm_page *) -1L)	/* [get only] */