linux-ext4 - Re: [PATCH 5 2/4] Return 32/64-bit dir name hash according to usage type

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Message-ID: <4F971A6A.90502@redhat.com>
Date:	Tue, 24 Apr 2012 16:26:02 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Andreas Dilger <adilger@...mcloud.com>
CC:	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>,
	linux-ext4@...r.kernel.org, Fan Yong <yong.fan@...mcloud.com>,
	bfields@...hat.com
Subject: Re: [PATCH 5 2/4] Return 32/64-bit dir name hash according to usage
 type

On 4/24/12 4:28 PM, Andreas Dilger wrote:
> On 2012-04-24, at 2:21 PM, Eric Sandeen wrote:
>> I know I'm being a little pedantic w/ the late review here....
>>
>> It seems like the only differences between ext4_dir_llseek and the old ext4_llseek are these:
>>
>> 1) For SEEK_END, we now return -EINVAL for a positive offset (i.e. past EOF)
>> 2) For SEEK_END, we seek to ext4_get_htree_eof() not to inode->i_size
>> 3) For SEEK_SET, we impose different limits for max offset
>>  - s_maxbytes / ext4_get_htree_eof for !dx/dx, vs. s_bitmap_maxbytes/s_maxbytes
>>
>> Do any of these changes relate to the hash collision problem?  Are any of them uniquely required for ext4, enough to warrant cut & paste of the vfs llseek code (again?)
>>
>> What I'm getting at is: what are the reasons that we cannot use generic_file_llseek_size(), maybe with a new argument to specify a non-standard location for SEEK_END.  Such a change would require a solid explanation, but it'd probably go in if it meant one less seek implementation to worry about.
> 
> So, when we were looking at this code, it makes sense that if dir seek is being done for telldir/seekdir that the parameters for ext4 are hash functions, so they should be compared against hash limits instead of the file size.
> 
> This probably makes sense for other filesystems that use hash cookies instead of byte offsets to have a similar dir seek implementation, but I thought there might be a controversy about this and I'm happy to get it into ext4 as a starting point.

That makes sense... but I think the generic code could be expanded to handle this, if we used the existing size parameter to specify max seekable offset in the dir, and a new parameter to cause SEEK_END to behave in a special way (not depending on i_size?)

-Eric

> Cheers, Andreas
> --
> Andreas Dilger                       Whamcloud, Inc.
> Principal Lustre Engineer            http://www.whamcloud.com/
> 
> 
> 
> 

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