SF_ORIG_PPN
ppn = SF_ORIG_PPN(coff);
#define SF_SAME_PPN(off) (SF_BUF_PPN(off) == SF_ORIG_PPN(off))
#define SF_DIFF_PPN(off) (SF_BUF_PPN(off) != SF_ORIG_PPN(off))
low_src_ppn = SF_ORIG_PPN(sfile.buf_offset);