struct buf {
  int flags;
  uint dev;
  // 64-bit, because the filesystem's block numbers are. This was a uint, which
  // capped any device at 2^32 blocks (16 TB at 4 KB) no matter what the format
  // above it could address -- SchFS is a 64-bit format and was being served by
  // a 32-bit cache.
  uint64 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