In this paper, we propose WALL, a novel writeback-aware LLC management scheme to reduce the number of LLC writebacks and consequently improve performance, energy efficiency, and lifetime of a PCM-based main memory system. First, we investigate the writeback behavior of LLC sets and show that writebacks are not uniformly distributed among sets; some sets observe much higher writeback rates than others. We then propose a writeback-aware set-balancing mechanism, which employs the underutilized LLC sets with few writebacks as an auxiliary storage for storing the evicted dirty lines of sets with frequent writebacks. We also propose a simple and effective writeback-aware replacement policy to avoid the eviction of the writeback blocks that are highly reused after being evicted from the cache. Our experimental results show that WALL achieves an average of 26.6% reduction in the total number of LLC writebacks, compared to the baseline scheme, which uses the LRU replacement policy. As a result, WALL can reduce the memory energy consumption by 19.2% and enhance PCM lifetime by 1.25x, on average, on an 8-core system with a 4GB PCM main memory, running memory intensive applications.