I've been creating a procedural material (white a bit glossy paint with rusty dents). I've created most of it without problems, but when I connect output of noise node (after few modifications) to displacement socket of material output, some of the dents are correct (downwards) and some of them are upwards. Is there a way to force it to be directed always in the opposite side of normal? (Make it always go downwards into the face)
If I can read your title with this much detail, if the displacement is different on different faces, then it sounds like your normals are not all facing the same direction. You can fix this by looking in the Mesh menu (in edit mode). Select all the faces, go to Mesh->Normals-> Recalculate Outside
This will attempt to flip any normals that are facing INTO the mesh, so that they face OUT of the mesh. FYI, this doesn't work very well on non-manifold meshes.