From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: RFC: REQ_PRIO use
Date: Tue, 20 Nov 2018 01:39:07 -0800
Message-ID: <20181120093907.GA32682@infradead.org>
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To: Damien Le Moal <Damien.LeMoal@wdc.com>
Cc: "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>, "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>, "dm-devel@redhat.com" <dm-devel@redhat.com>
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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..