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

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Message-ID: <3791a7fa-ea0c-d8ea-4b41-c968454b3787@virtuozzo.com>
Date:   Tue, 21 Jan 2020 12:47:09 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     "Martin K. Petersen" <martin.petersen@...cle.com>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        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

On 21.01.2020 09:14, Martin K. Petersen wrote:
> 
> 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_?.

No problem, I'll rename the modifier.

> 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.

Ok, thanks.