FreeBSDのtopのvnreadとはなにか？

bwait(bp, PVM, "vnread");

void
bwait(struct buf *bp, u_char pri, const char *wchan)
{

        mtx_lock(&bdonelock);
        while ((bp->b_flags & B_DONE) == 2)
                msleep(bp, &bdonelock, pri, wchan, 0);
        mtx_unlock(&bdonelock);
}

#define B_AGE           0x00000001      /* Move to age queue when I/O done.
						   */
#define B_NEEDCOMMIT    0x00000002      /* Append-write in progress. */
#define B_ASYNC         0x00000004      /* Start I/O, do not wait. */
#define B_DIRECT        0x00000008      /* direct I/O flag (pls free vmio)
						 */
#define B_DEFERRED      0x00000010      /* Skipped over for cleaning */
#define B_CACHE         0x00000020      /* Bread found us in the cache. */
#define B_VALIDSUSPWRT  0x00000040      /* Valid write during suspension. */
#define B_DELWRI        0x00000080      /* Delay I/O until buffer reused. */
#define B_PERSISTENT    0x00000100      /* Perm. ref'ed while EXT2FS
#define B_DONE          0x00000200      /* I/O completed. */