linux-kernel - Re: [PATCH 3/4] loop: Add 'lo_logical

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Message-ID: <25f0e1a4-f211-670e-00f8-364b3a85b468@suse.de>
Date:   Tue, 1 Nov 2016 17:57:08 +0100
From:   Hannes Reinecke <hare@...e.de>
To:     Christoph Hellwig <hch@....de>
Cc:     Jens Axboe <axboe@...com>, Alexander Graf <agraf@...e.com>,
        Ming@...e.de, "\"Lei" <"tom.leiming\""@gmail.co>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] loop: Add 'lo_logical_blocksize'

On 11/01/2016 03:01 PM, Christoph Hellwig wrote:
> On Mon, Oct 31, 2016 at 08:37:18PM +0100, Hannes Reinecke wrote:
>> Add a new field 'lo_logical_blocksize' to hold the logical
>> blocksize of the loop device.
>
> Why do we use the same flag for both block sizes?  If there is a good
> reason for that it should be documented and both should be in the same
> patch.
>
_Actually_ the flag enables you to set the logical blocksize.
However, if you want to set the logical blocksize you also have to set a 
matching physical blocksize.
And the physical blocksize itself isn't settable, rather taken from the 
underlying device/filesystem/whatever.
So the flag itself does not have a dual purpose, it's just out of 
necessity that you have to set the physical blocksize if you want to 
change the logical blocksize.

And I got pushback when I proposed to set the physical blocksize always,
so I made it dependent on the LO_BLOCKSIZE flag, too.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@...e.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)