kernel/vm: Don't invoke compute_area_page_commitment in map_backing_store.

It seems this is still too early and not enough fields in the VMArea
have been initialized. Instead just check the protection manually.

Also fix some more locking behavior in the error path.

src/system/kernel/vm/vm.cpp

@@ -1264,18 +1264,16 @@ map_backing_store(VMAddressSpace* addressSpace, VMCache* cache, off_t offset,
 		cache = newCache;
 	}
 
-	// attach the cache to the area
-	area->cache = cache;
-	area->cache_offset = offset;
-
-	// point the cache back to the area
-	cache->InsertAreaLocked(area);
-
 	if ((flags & CREATE_AREA_DONT_COMMIT_MEMORY) == 0) {
-		const size_t commitmentPages = compute_area_page_commitment(area);
-		status = cache->SetMinimalCommitment(commitmentPages * B_PAGE_SIZE, priority);
-		if (status != B_OK)
-			goto err2;
+		uint32 commitProtection = B_WRITE_AREA | B_KERNEL_WRITE_AREA;
+		if (cache->source == NULL)
+			commitProtection |= B_READ_AREA | B_KERNEL_READ_AREA;
+
+		if ((protection & commitProtection) != 0) {
+			status = cache->SetMinimalCommitment(size, priority);
+			if (status != B_OK)
+				goto err2;
+		}
 	}
 
 	// check to see if this address space has entered DELETE state
@@ -1310,6 +1308,12 @@ map_backing_store(VMAddressSpace* addressSpace, VMCache* cache, off_t offset,
 	if (status != B_OK)
 		goto err2;
 
+	// attach the cache to the area
+	area->cache = cache;
+	area->cache_offset = offset;
+
+	// point the cache back to the area
+	cache->InsertAreaLocked(area);
 	if (mapping == REGION_PRIVATE_MAP)
 		cache->Unlock();
 
@@ -1328,21 +1332,24 @@ map_backing_store(VMAddressSpace* addressSpace, VMCache* cache, off_t offset,
 	return B_OK;
 
 err3:
-	cache->Lock();
+	if (mapping != REGION_PRIVATE_MAP)
+		cache->Unlock();
+	cache->RemoveArea(area);
+	area->cache = NULL;
+	if (mapping != REGION_PRIVATE_MAP)
+		cache->Lock();
+
 	addressSpace->RemoveArea(area, allocationFlags);
 err2:
-	cache->Unlock();
-	cache->RemoveArea(area);
-	cache->Lock();
-	area->cache = NULL;
-
 	if (mapping == REGION_PRIVATE_MAP) {
 		// We created this cache, so we must delete it again. Note, that we
 		// need to temporarily unlock the source cache or we'll otherwise
 		// deadlock, since VMCache::_RemoveConsumer() will try to lock it, too.
-		sourceCache->Unlock();
-		cache->ReleaseRefAndUnlock();
-		sourceCache->Lock();
+		if (sourceCache != cache)
+			sourceCache->Unlock();
+		cache->ReleaseRef();
+		if (sourceCache != cache)
+			sourceCache->Lock();
 	}
 err1:
 	addressSpace->DeleteArea(area, allocationFlags);