When vm_fault_copy_entry() is called from vm_map_protect() for a wired

entry and performs the upgrade of the entry permissions from read-only
to read-write, we must allow to search for the source pages in the
backing object, like we do in the case of forking the read-only wired
entry. For the fork case, the behaviour is allowed by src_readonly
boolean, which in fact is only used to assert that read-write case
provides all source pages in the top-level object.

Eliminate the src_readonly variable.  Allow for the copy loop to look
into the backing objects, add explicit asserts to ensure that only
read-only and upgrade case actually does.

Expand comments. Change the panic call into assert.

Reported by:	markj
Tested by:	markj, pho (previous version)
Reviewed by:	alc
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

1 files changed, 15 insertions, 8 deletions

diff --git a/sys/vm/vm_fault.c b/sys/vm/vm_fault.c
index ba7692c..3faf28b 100644
--- a/sys/vm/vm_fault.c
+++ b/sys/vm/vm_fault.c
@@ -1240,7 +1240,7 @@ vm_fault_copy_entry(vm_map_t dst_map, vm_map_t src_map,
 	vm_offset_t vaddr;
 	vm_page_t dst_m;
 	vm_page_t src_m;
-	boolean_t src_readonly, upgrade;
+	boolean_t upgrade;
 
 #ifdef	lint
 	src_map++;
@@ -1250,7 +1250,6 @@ vm_fault_copy_entry(vm_map_t dst_map, vm_map_t src_map,
 
 	src_object = src_entry->object.vm_object;
 	src_pindex = OFF_TO_IDX(src_entry->offset);
-	src_readonly = (src_entry->protection & VM_PROT_WRITE) == 0;
 
 	/*
 	 * Create the top-level object for the destination entry. (Doesn't
@@ -1321,25 +1320,33 @@ vm_fault_copy_entry(vm_map_t dst_map, vm_map_t src_map,
 
 		/*
 		 * Find the page in the source object, and copy it in.
-		 * (Because the source is wired down, the page will be in
-		 * memory.)
+		 * Because the source is wired down, the page will be
+		 * in memory.
 		 */
 		VM_OBJECT_RLOCK(src_object);
 		object = src_object;
 		pindex = src_pindex + dst_pindex;
 		while ((src_m = vm_page_lookup(object, pindex)) == NULL &&
-		    src_readonly &&
 		    (backing_object = object->backing_object) != NULL) {
 			/*
-			 * Allow fallback to backing objects if we are reading.
+			 * Unless the source mapping is read-only or
+			 * it is presently being upgraded from
+			 * read-only, the first object in the shadow
+			 * chain should provide all of the pages.  In
+			 * other words, this loop body should never be
+			 * executed when the source mapping is already
+			 * read/write.
 			 */
+			KASSERT((src_entry->protection & VM_PROT_WRITE) == 0 ||
+			    upgrade,
+			    ("vm_fault_copy_entry: main object missing page"));
+
 			VM_OBJECT_RLOCK(backing_object);
 			pindex += OFF_TO_IDX(object->backing_object_offset);
 			VM_OBJECT_RUNLOCK(object);
 			object = backing_object;
 		}
-		if (src_m == NULL)
-			panic("vm_fault_copy_wired: page missing");
+		KASSERT(src_m != NULL, ("vm_fault_copy_entry: page missing"));
 		pmap_copy_page(src_m, dst_m);
 		VM_OBJECT_RUNLOCK(object);
 		dst_m->valid = VM_PAGE_BITS_ALL;