linux-kernel - Re: sget() misuse in nilfs

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Date:	Thu, 07 May 2009 01:09:34 +0900 (JST)
From:	Ryusuke Konishi <ryusuke@...g.net>
To:	viro@...IV.linux.org.uk
Cc:	konishi.ryusuke@....ntt.co.jp, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: sget() misuse in nilfs

Hi,
On Wed, 06 May 2009 15:28:59 +0900 (JST), Ryusuke Konishi wrote:
> If we treat read-only mount as the latest snapshot at the time (though
> we didn't take this interpretation), the transitions can be reduced
> to:
> 
>  * r/w -> r/w.  Allowed.
>  * r/w -> snapshot.  Allowed if no checkpoint number was given (or the
>                      latest checkpoint was specified)
>  * snapshot -> r/w.  Allowed if it's the latest one and no r/w is there.
>  * snapshot -> snapshot.  Only if it's the same.
> 
> Right?

Ah, I had forgotten garbage collection (GC).

GC can break checkpoints which are not marked as snapshot.  ro-mount
cannot coexist with rw-mount because GC works while an rw-mount is
there.

Sorry, the above interpretation was not easily realized.
 
> But it still needs test_exclusive_mount().
> 
> The test_exclusive_mount() may be eliminable by adding rw-mount-exists
> flag on the_nilfs struct.  I'll take some thinking.

The elimination of test_exclusive_mount() was possible by this method
if we can treat ro-mount as the latest checkpoint at the time.

I'd like to consider if a similiar elimination is possible in case
that ro-mount and rw-mount cannot coexist.

Regards,
Ryusuke Konishi
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