[dm-devel] RFC: REQ_PRIO use

[Damien Le Moal]

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