linux-kernel - Re: [PATCH block v2 2/3] block: Add support for REQ

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Date:   Tue, 21 Jan 2020 01:14:05 -0500
From:   "Martin K. Petersen" <martin.petersen@...cle.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        martin.petersen@...cle.com, axboe@...nel.dk, tytso@....edu,
        adilger.kernel@...ger.ca, Chaitanya.Kulkarni@....com,
        darrick.wong@...cle.com, ming.lei@...hat.com, osandov@...com,
        jthumshirn@...e.de, minwoo.im.dev@...il.com, damien.lemoal@....com,
        andrea.parri@...rulasolutions.com, hare@...e.com, tj@...nel.org,
        ajay.joshi@....com, sagi@...mberg.me, dsterba@...e.com,
        bvanassche@....org, dhowells@...hat.com, asml.silence@...il.com
Subject: Re: [PATCH block v2 2/3] block: Add support for REQ_NOZERO flag

Kirill,

> +	if (flags & BLKDEV_ZERO_NOUNMAP)
> +		req_flags |= REQ_NOUNMAP;
> +	if (flags & BLKDEV_ZERO_ALLOCATE)
> +		req_flags |= REQ_NOZERO|REQ_NOUNMAP;

I find there is some dissonance between using BLKDEV_ZERO_ALLOCATE to
describe this operation in one case and REQ_NOZERO in the other.

I understand why not zeroing is important in your case. However, I think
the allocation aspect is semantically more important. Also, in the case
of SCSI, the allocated blocks will typically appear zeroed. So from that
perspective REQ_NOZERO doesn't really make sense. I would really prefer
to use REQ_ALLOCATE to describe this operation. I agree that "do not
write every block" is important too. I just don't have a good suggestion
for how to express that as an additional qualifier to REQ_ALLOCATE_?.

Also, adding to the confusion: In the context of SCSI, ANCHOR requires
UNMAP. So my head hurts a bit when I read REQ_NOZERO|REQ_NOUNMAP and
have to translate that into ANCHOR|UNMAP.

Longer term, I think we should consider introducing REQ_OP_SINGLE_RANGE
or something like that as an umbrella operation that can be used to
describe zeroing, allocating, and other things that operate on a single
LBA range with no payload. Thus removing both the writiness and the
zeroness from the existing REQ_OP_WRITE_ZEROES conduit.

Naming issues aside, your patch looks fine. I'll try to rebase my SCSI
patches on top of your series to see how things fit.

-- 
Martin K. Petersen	Oracle Linux Engineering