linux-kernel - Re: [PATCH] Allow userspace block device implementation

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Date:	Tue, 28 Jul 2009 18:56:35 -0400
From:	Theodore Tso <tytso@....edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Kyle Moffett <kyle@...fetthome.net>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Zachary Amsden <zamsden@...hat.com>, Tejun Heo <tj@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, axboe@...nel.dk, hch@...radead.org,
	akpm@...ux-foundation.org, Paul.Clements@...eleye.com,
	miklos <miklos@...redi.hu>
Subject: Re: [PATCH] Allow userspace block device implementation

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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