MAHOVLJANI is a village in Bosnia near the town of BANJA LUKA , KALABA were ethnic Serbian family in this village and they did not live in Serbia. :)
There are two branches of this family, with different patron saints ("slava") - did your father ever mention this and do you know which saint his family celebrated? That information would make researching the surname much easier.
I will look for information on your family and where the records are available. So far found that MAHOVLJANI had its own Orthodox parish , so these are the church records your ancestors were written into (author "pravoslavna crkva", ). You should inquire through Banja Luka eparchy about these records:
Саборни храм Свете Тројице у Бањалуци,
78000 Бања Лука, Алеја Светог Саве 5,
tel:+387 51 348-740; + 387 51 348-751
The civil transcripts of these vital records , available for period after around 1875. should be in the LAKTAŠI municipal office ("matični ured") and more documents in the historical archive in BANJA LUKA.
If your grandparents died during WWII there is information in commission reports, documents and testimonies. The village is part of mountain KOZARA area known as POTKOZARJE - these villages and civilians who lived there suffered great loses during the WWII.
My email is yugaya at gmail dot com if you have more questions.