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

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Date:	Tue, 28 Jul 2009 13:50:56 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	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,
	tytso@....edu, miklos <miklos@...redi.hu>
Subject: Re: [PATCH] Allow userspace block device implementation

On Tue, 28 Jul 2009, Andi Kleen wrote:

> Kyle Moffett <kyle@...fetthome.net> writes:
> >
> >   (1) The ability to rearrange, resize, and restructure
> > partition-tables on the fly.  The existing "re-read partition tables"
> > infrastructure does not safely and reasonably handle changes to the
> > partition-table while partitions are mounted. 
> 
> It doesn't today (and I really hate it too), but is there a hard reason it
> couldn't be fixed to support that properly? 

If something has a partition open (and it doesn't really even have to be a 
mounted filesystem, altough that's obviously the most relevant case), how 
can you reasonably change the partition from underneath it? So I assume 
you mean that partitions were opened earlier (for a mount) would not be 
touched.

And these days, that _should_ just work. The "reread partition table" 
operation should just leave the old bdev's around (so a mounted filesystem 
simply won't _see_ the new partitions, but will continue to use the old 
one), and for all I know that might even work these days.

[ Here "these days" is admittedly only in comparison to the _original_ 
 Linux code, which used block numbers. Many years ago. ]

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.

So I think re-reading the partition tables should be safe these days. It 
definitely didn't _use_ to be the case due to dev_t issues, but that's 
really ancient.

It may be that we just have the old check in place ("don't allow 
re-reading if something has mounted a partition"), and we could just get 
rid of it. I have not looked. 

But if you actually meant that re-reading the partition table should 
_change_ a "struct block_dev" that is in use, then I think that would be a 
bad idea. At the very least, it should involve a re-mount or something.

		Linus
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