Can anyone plz explain me the fields

Can anyone plz explain me the fields skb = kmem_cache_alloc(skbuff_head_cache, gfp_mask & ~_ _GFP_DMA); ... ... ... size = SKB_DATA_ALIGN(size); data = kmalloc(size + sizeof(struct skb_shared_info), gfp_mask); It is the simplified version of skb_alloc().