We have two places that check the flag, the writeback tracking code: block/blk-wbt.c:#define REQ_HIPRIO (REQ_SYNC | REQ_META | REQ_PRIO) block/blk-wbt.c: if ((rw & REQ_HIPRIO) || wb_recent_wait(rwb) || current_is_kswapd()) and bcache: drivers/md/bcache/request.c: !(bio->bi_opf & REQ_PRIO)) drivers/md/bcache/request.c: !(bio->bi_opf & REQ_PRIO) && the original idea was that not all I/O is a priority we should not blindly boost priority for metadata. Of course wbt actually ignores and it and we may just give in and just check REQ_META, except for the fact that f2fs now actually submits REQ_PRIO requests without REQ_META..
