linux-kernel - Re: is the inode an orphan?

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Message-ID: <20071030141004.GA27039@atrey.karlin.mff.cuni.cz>
Date:	Tue, 30 Oct 2007 15:10:04 +0100
From:	Jan Kara <jack@...e.cz>
To:	Artem Bityutskiy <dedekind@...dex.ru>
Cc:	Al Viro <viro@....linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: is the inode an orphan?

> Al Viro wrote:
> >Define orphan.  It might very well be still opened after the only link
> >to it had been removed and you still will get IO on it.
> 
> Well, in the mail I called files like open/unlink the last link/do some I/O 
> orphans. Let me shortly describe the problem I'm trying to solve.
> 
> In our FS when we're in ->unlink() and i_nlink becomes 0, we have to record 
> this inode in the table of orphans, and remove it from there in 
> ->delete_inode(). This is needed to be able to dispose of orphans in case 
> of an unclean reboot on the next mount. AFAIK, ext3 has something similar. 
> I just figured that this could be optimized - in most cases 
> ->delete_inode() is called right after ->unlink(), and I wanted to avoid 
> putting the inode to the orphan table in those cases.
  Yes, ext3 has something similar. But actually ext3 would have to insert
inode in the orphan list anyway - in delete_inode we do truncate and
for it we also insert the inode into the orphan list because truncate
can be too large to fit into a single transaction.

> I.e., if one just does "unlink file", then it is not going to be an orphan. 
> And most cases are like this. It is rather rare to open a file, unlink it, 
> and keep utilizing it.
> 
> So my question was - while I'm in ->unlink(), how do I figure out that this 
> is not an orphan. So I was thinking about
> 
> if (atomic_read(&inode->i_count) == 2)
> 
> then this is not an orphan and ->delete_inode() will be called straight 
> away (i_nlink is assumed to be 0).
> 
> But I've now also figured that ->unlink() may race with write-back, and 
> there might be a write-back I/O between ->unlink() (and during it) and 
> ->delete_inode(), even though the user-space does not have the file in 
> question opened.
> 
> So, at the moment, AFAIU
> 
> if (atomic_read(&inode->i_count) == 2 && !(inode->i_state & I_DIRTY))
> 
> then there won't be any I/O on the inode between ->unlink() and 
> ->delete_inode i_nlink is assumed to be 0). Is that right, safe and 
> acceptable to use such checks in ->unlink() for optimization?
> 
> P.S. the code and short description of the FS I refer is here: 
> http://www.linux-mtd.infradead.org/doc/ubifs.html
  Hmm, I'm just not sure whether unlink cannot somehow race with open
(at least I don't see any lock that would prevent open while unlink is
in progress)...

									Honza
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