struct buf {
    int flags;
    uint dev;
    uint blockno;
    struct sleeplock lock;
    uint refcnt;
    struct buf *prev; // LRU cache list
    struct buf *next;
    struct buf *qnext; // disk queue
    uchar data[BSIZE];
};
#define B_VALID 0x2  // buffer has been read from disk
#define B_DIRTY 0x4  // buffer needs to be written to disk

