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Date:	Thu, 4 Aug 2011 11:25:19 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Allison Henderson <achender@...ux.vnet.ibm.com>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 1/1 v3] ext4: fix xfstests 75, 112, 127 punch hole
 failure

On Thu, Aug 04, 2011 at 12:22:58AM -0700, Allison Henderson wrote:
> 
> Oh, I think we do avoid calling the unmap for this last condition
> though.  The first and last page offsets are calculated earlier for
> calling truncate_inode_pages_range to release all the pages in the
> hole. The idea is that everything from first_page_offset to
> last_page_offset covers all the page aligned pages in the hole.  So
> then if offset and length are aligned, we basically end up with
> first_page_offset = offset and last_page_offset = offset + length,
> and the page_len will turn out to be zero.  Right math?  Maybe we
> can add some comments or something to help clarify.

Yeah, sorry, I wasn't clear enough about the condition.  Consider the
situation where we punch the region:

   4092 -- 8197

In the previous section of code, we would zero out the byte ranges
4092--4095 and 8192--8197.  What's left is a completely page-aligned
range, which would have already been taken care of already.  But since
we're calculating based on offsets, I believe there will be an
unnecessary call to ext4_unmap_page_range().  

BTW, the name ext4_unmap_page_range() is a bit confusing; maybe we
should rename it to ext4_unmap_partial_page_buffers()?

I know you were copying from the ext4_block_zero_page_range() function
and its calling sequence (but in my opinion that function wasn't named
well and the comments in that code aren't good either).

I also wonder why we can't fold the functionality found in
ext4_unmap_page_range() into ext4_block_zero_page_range().  Did you
look into that option?

Regards,

						- Ted
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