linux-kernel - Re: [patch 14/23] inodes: Support generic defragmentation

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Message-ID: <20071107103124.GD7374@lazybastard.org>
Date:	Wed, 7 Nov 2007 11:31:24 +0100
From:	Jörn Engel <joern@...fs.org>
To:	Jörn Engel <joern@...fs.org>
Cc:	Christoph Lameter <clameter@....com>, akpm@...ux-foundatin.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Mel Gorman <mel@...net.ie>
Subject: Re: [patch 14/23] inodes: Support generic defragmentation

On Wed, 7 November 2007 11:17:48 +0100, Jörn Engel wrote:
> > +/*
> > + * Function for filesystems that embedd struct inode into their own
> > + * structures. The offset is the offset of the struct inode in the fs inode.
> > + */
> > +void *fs_get_inodes(struct kmem_cache *s, int nr, void **v,
> > +						unsigned long offset)
> > +{
> > +	int i;
> > +
> > +	for (i = 0; i < nr; i++)
> > +		v[i] += offset;
> > +
> > +	return get_inodes(s, nr, v);
> > +}
> > +EXPORT_SYMBOL(fs_get_inodes);
> 
> The fact that all pointers get changed makes me a bit uneasy:
> 	struct foo_inode v[20];
> 	...
> 	fs_get_inodes(..., v, ...);
> 	...
> 	v[0].foo_field = bar;
> 	
> No warning, but spectacular fireworks.
> 
> > +void kick_inodes(struct kmem_cache *s, int nr, void **v, void *private)
> > +{
> > +	struct inode *inode;
> > +	int i;
> > +	int abort = 0;
> > +	LIST_HEAD(freeable);
> > +	struct super_block *sb;
> > +
> > +	for (i = 0; i < nr; i++) {
> > +		inode = v[i];
> > +		if (!inode)
> > +			continue;
> 
> NULL is legal here?  Then fs_get_inodes should check for NULL as well
> and not add the offset to NULL pointers, I guess.

Ignore these two comments.  Reading further before making them would
have helped. ;)

Jörn

-- 
Fancy algorithms are slow when n is small, and n is usually small.
Fancy algorithms have big constants. Until you know that n is
frequently going to be big, don't get fancy.
-- Rob Pike
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