xv6-riscv-kernel/kernel/buf.h

struct buf {
  int              valid; // has data been read from disk?
  int              disk;  // does disk "own" buf?
  u32              dev;
  u32              blockno;
  struct sleeplock lock;
  u32              refcnt;
  struct buf      *prev; // LRU cache list
  struct buf      *next;
  u8               data[BSIZE];
};