linux-kernel - Re: [dm-devel] [RFC PATCH] init: boot to device-mapper targets without an initr*

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Date:	Wed, 12 May 2010 16:41:10 -0500
From:	Will Drewry <wad@...omium.org>
To:	agk@...hat.com
Cc:	dm-devel@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [dm-devel] [RFC PATCH] init: boot to device-mapper targets 
	without an initr*

On Wed, May 12, 2010 at 3:58 PM, Alasdair G Kergon <agk@...hat.com> wrote:
> On Wed, May 12, 2010 at 03:30:54PM -0500, Will Drewry wrote:
>> The format is dm=minor,rwmode,begin,length,target,target,params,with,commas
>
> 1. If we go down this route, pick a format that gives access to the full
> set of allowed table formats (or which can be extended trivially to do
> so in future).  So cope with multi-line tables e.g. with a marker (empty
> field?) to start a new table line.

Sounds good.  That shouldn't add any serious code complexity either.
An empty field (,,) would probably work nicely.

> 2. It would be wise to define a name and optional uuid too, so the device is
> fully visible and accessible from userspace through existing mechanisms.
> Originally the name+uuid were decoupled from the mapped device itself,
> but that got changed, though the code layout (dm-ioctl.c vs. dm.c) still
> makes them appear to be separate.

It wasn't clear how I could define those externally to the dm-ioctl code.

>> +     char target[24];
>> +     char target_params[256];
>
> I'm never a fan of hard-coded restrictions, but at least make them
> explicit as #defines, so people can see at a glance what to change
> if they get errors for exceeding them.

Agreed. I'll add defines or see if I can use the existing ones.
(Alternately, it might be possible to use the passed in string in place
 depending on the changes.)

>> +static void __init dm_setup_drive(void)
>
> Arguably most of this code could be a helper function exported from drivers/md.

True, or I could follow the do_mounts_md model and just use the ioctl interface.
Would a parallel function to ctl_ioctl which behaved the same but didn't expect
user-sourced dm_ioctl struct be a reasonable compromise?

That'd limit the code to parsing the parameter line and calling through without
duplicating the code in dev_create and table_load to yield the same behavior.

thanks!
will
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