linux-ext4 - Re: [PATCH] vfs: allow custom EOF in generic_file

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Date:	Fri, 27 Apr 2012 11:28:58 -0500
From:	Eric Sandeen <sandeen@...deen.net>
To:	Eric Sandeen <sandeen@...hat.com>
CC:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	ext4 development <linux-ext4@...r.kernel.org>,
	Andreas Dilger <adilger@...ger.ca>,
	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
Subject: Re: [PATCH] vfs: allow custom EOF in generic_file_llseek code

On 4/27/12 11:21 AM, Eric Sandeen wrote:
> For ext3/4 htree directories, using the vfs llseek function with
> SEEK_END goes to i_size like for any other file, but in reality
> we want the maximum possible hash value.  Recent changes
> in ext4 have cut & pasted generic_file_llseek() back into fs/ext4/dir.c,
> but replicating this core code seems like a bad idea, especially
> since the copy has already diverged from the vfs.
> 
> This patch implements a version of generic_file_llseek which can accept
> both a custom maximum offset, and a custom EOF position.  With this
> in place, ext4_dir_llseek can pass in the appropriate maximum hash
> position for both maxsize and eof, and get what it wants.
> 
> As far as I know, this does not fix any bugs - nfs in the kernel
> doesn't use SEEK_END, and I don't know of any user who does.  But
> some ext4 folks seem keen on doing the right thing here, and I can't
> really argue.
> 
> (Patch also fixes up some comments slightly)
> 
> Signed-off-by: Eric Sandeen <sandeen@...hat.com>

I guess I should ahev done a patch series, although the ext4 patch is so
messy it's hard to read as a patch.  With the new framework in place,
ext4_dir_llseek can just be:

/*
 * ext4_dir_llseek() calls generic_file_llseek_size to handle htree
 * directories, where the "offset" is in terms of the filename hash
 * value instead of the byte offset.
 *
 * Because we may return a 64-bit hash that is well beyond offset limits,
 * we need to pass the max hash as the maximum allowable offset in
 * the htree directory case.
 *
 * For non-htree, ext4_llseek already chooses the proper max offset.
 */
loff_t ext4_dir_llseek(struct file *file, loff_t offset, int origin)
{
        struct inode *inode = file->f_mapping->host;
        int dx_dir = is_dx_dir(inode);
        loff_t htree_max = ext4_get_htree_eof(file);

        if (likely(dx_dir))
                return generic_file_llseek_size_eof(file, offset, origin,
                                                    htree_max, htree_max);
        else
                return ext4_llseek(file, offset, origin);
}


-Eric

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