linux-kernel - Re: [PATCH] init: add root=PARTUUID=UUID+/-%d support

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Message-ID: <BANLkTimi2o4vnSWp5rQGrU3BTSC+YyO0qg@mail.gmail.com>
Date:	Thu, 2 Jun 2011 14:49:44 +0200
From:	Kay Sievers <kay.sievers@...y.org>
To:	Will Drewry <wad@...omium.org>
Cc:	linux-kernel@...r.kernel.org, Jens Axboe <jaxboe@...ionio.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Namhyung Kim <namhyung@...il.com>,
	Trond Myklebust <Trond.Myklebust@...app.com>
Subject: Re: [PATCH] init: add root=PARTUUID=UUID+/-%d support

On Wed, Jun 1, 2011 at 21:49, Will Drewry <wad@...omium.org> wrote:
> Not sure if this is practical and/or interesting to others, but it's a
> minimal change --
>
> Expands root=PARTUUID=UUID syntax to support selecting a root partition
> by integer offset from a known, unique partition.  This approach
> provides similar properties to specifying a device and partition number,
> but using the UUID as the unique path prior to evaluating the offset.
>
> For example,
>  root=PARTUUID=99DE9194-FC15-4223-9192-FC243948F88B+1
> selects the partition with UUID 99DE.. then select the next
> partition.
>
> This change is motivated by a particular usecase in Chromium OS where
> the bootloader can easily determine what partition it is on (by UUID)
> but doesn't perform general partition table walking.
>
> That said, support for this approach provides a direct mechanism for the
> user to modify the root partition to boot without specifically needing
> to extract each PARTUUID or update the bootloader explicitly when the
> root partition UUID is changed (if it is recreated to be larger, for
> instance, restored, etc).  Pinning to a /boot partition UUID with an
> offset allows the arbitrary root partition reconfiguration/modifications
> with slightly less ambiguity than just [dev][partition] and less
> stringency than the specific root partition UUID.

Kind of a creative notation to add/substract something from a uuid. :)

If we are not sure we'll not need another magic thing in the future,
we might want to go for a named parameter to add?
Maybe instead of:
  root=PARTUUID=99DE9194-FC15-4223-9192-FC243948F88B+2
we could do:
  root=PARTUUID=99DE9194-FC15-4223-9192-FC243948F88B,PARTNROFF=2

Might be it's a but over the top, but the key is named PARTUUID, and a
+- to it doesn't seem to explain what it does and can not really be
expanded for even more creative uses. :)

Kay
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