include/linux/blk_types.h states that REQ_PRIO intended use is to boost a request priority in cfq: __REQ_PRIO, /* boost priority in cfq */ REQ_PRIO is used by different file systems (ext4, btrfs, f2fs, gfs2, erofs) and a couple of dm drivers (bcache and dm-zoned), but nobody actually acts on the flag being set, certainly not cfq since this scheduler is now gone in 4.21. I think this mandates some cleanups. I can see 2 options: 1) Have bfq act on this flag 2) Remove it entirely and replace its use with explicit setting of BIOs priority values, either directly through the struct bio bi_ioprio field or indirectly through setting the issuer task contecxt iopriority or eventually by adding a bi_ionice field to struct bio. For instance, ext4 sets its journald task context I/O priority, which result in all BIOs issued by that task to get an explicit I/O priority value. If option (2) is the preferred choice, looking at the current use of REQ_PRIO, removing this flag does not seem difficult at all. In this case, changes by the current users to explicitly set BIO I/O priorities can be done later with different patches. Comments, thoughts ? Best regards. -- Damien Le Moal Western Digital Research
