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Date: Tue, 28 Jul 2009 18:56:35 -0400
From: Theodore Tso <tytso@mit.edu>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andi Kleen <andi@firstfloor.org>, Kyle Moffett <kyle@moffetthome.net>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>,
       Zachary Amsden <zamsden@redhat.com>, Tejun Heo <tj@kernel.org>,
       Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
       axboe@kernel.dk, hch@infradead.org, akpm@linux-foundation.org,
       Paul.Clements@steeleye.com, miklos <miklos@szeredi.hu>
Subject: Re: [PATCH] Allow userspace block device implementation
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On Tue, Jul 28, 2009 at 01:50:56PM -0700, Linus Torvalds wrote:
> 
> Filesystems long ago _used_ to index things by device number and block - 
> and that meant that re-reading partition tables was _really_ dangerous, 
> because the "device number" would just magically mean something else for a 
> mounted filesystem. But we've indexed things by bdev for a longish time 
> now, and most (all?) filesystems use "sb_bread()" instead of bread etc.

Filesystems don't, but some userspace programs do depend on the dev_t
returned by stat to uniquely identify a mounted filesystem.  (And it's
guaranteed by POSIX).  So what this means is that if we're going to
allow re-reading the partition table, we should (a) avoid changing the
dev_t used by any mounted filesystem, and (b) we should either assign
a new dev_t for any new partitions, or we should disallow mounting a
filesystem with a new dev_t already in use by an already mounted
filesystem with the same dev_t before the partition table was
reorganized.

						- Ted
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