PRSHIFT           120 fs/xfs/linux-2.6/xfs_buf.c #define BTOPR(b)	(((unsigned int)(b) + (NBPPR - 1)) >> PRSHIFT)
PRSHIFT           121 fs/xfs/linux-2.6/xfs_buf.c #define BTOPRT(b)	(((unsigned int)(b) >> PRSHIFT))