[Ocfs2-devel] [PATCH 3/3] ocfs2/xattr: Proper hash collision handle in bucket division.v2

[Joel Becker]

On Fri, Oct 24, 2008 at 05:02:28PM +0800, Tao Ma wrote:
>
>
> Joel Becker wrote:
>> On Fri, Oct 24, 2008 at 12:43:53AM -0700, Joel Becker wrote:
>>> On Fri, Oct 24, 2008 at 02:15:35PM +0800, Tao Ma wrote:
>>>> Joel Becker wrote:
>>>>> 	I have an idea to do what you're doing, but cleaner.  I'll look
>>>>> at cmp_xe and try again in the morning.
>>>> cool, so waiting for your perfect code. ;)
>>> Zing!  You got me :-)
>> 	How's this (untested)?
>> /*
>>  * If this ocfs2_xattr_header covers more than one hash value, find a
>>  * place where the hash value changes.  Try to find the most even split.
>>  * The most common case is that all entries have different hash values,
>>  * and the first check we make will find a place to split.
>>  */
>> static int ocfs2_xattr_find_divide_pos(struct ocfs2_xattr_header *xh)
>> {
>> 	struct ocfs2_xattr_entry *entries = &xh->xh_entries;
>> 	int count = le16_to_cpu(xh->xh_count);
>> 	int delta, middle = count / 2;
>> 	/*  	 * We start at the middle.  Each step gets farther away in both
>> 	 * directions.  We therefore hit the change in hash value
>> 	 * nearest to the middle.  Note that this loop does not execute for
>> 	 * count < 2.
>> 	 */
>> 	for (delta = 0; delta < middle; delta++) {
>> 		/* Let's check delta earlier than middle */
>> 		if (!cmp_ex(&entries[middle - delta - 1], 			    &entries[middle - 
>> delta]))
>> 			return middle - delta;
>> 		/* For even counts, don't walk off the end */
>> 		if ((middle + delta + 1) == count)
>> 			continue;
>> 		/* Now try delta past middle */
>> 		if (!cmp_ex(&entries[middle + delta],
>> 			    &entries[middle + delta + 1]))
>> 			return middle + delta + 1;
>> 	}
>> 	/* Every entry had the same hash */
>> 	return count;
>> }
> yeah, this looks perfect and more efficient.
> I will modify my patch and then test it.

	Haha, it has a bug!  Those cmp_ex() calls should not have the
'!' in front of them.  We're looking for differences, not matches.  Take
out those '!'s.

Joel (sorry about the bug)

-- 

"Every new beginning comes from some other beginning's end."

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker at oracle.com
Phone: (650) 506-8127