typedef struct buftag {
   RelFileNode rnode;     /* physical relation identifier */
   ForkNumber  forkNum;
   BlockNumber blockNum;  /* relative to start of reln */
} BufferTag;

typedef struct BufferDesc { (simplified)
   BufferTag tag;        // ID of page contained in buffer
   int       buf_id;     // buffer's index number (from 0)
   Bits32    state;      // dirty, refcount, usage
   int       freeNext;   // link in freelist chain
   ...                   // others related to concurrency
} BufferDesc;