From: Allison Henderson <achender@linux.vnet.ibm.com>
Subject: Re: [PATCH 1/1 v3] ext4: fix xfstests 75, 112, 127 punch hole failure
Date: Thu, 04 Aug 2011 11:03:41 -0700
Message-ID: <4E3ADEFD.30501@linux.vnet.ibm.com>
References: <4E396744.2030003@linux.vnet.ibm.com> <20110804005001.GI3150@thunk.org> <4E3A48D2.3070905@linux.vnet.ibm.com> <20110804152519.GJ3150@thunk.org> <1312479840.7732.7.camel@mingming-laptop>
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Cc: "Ted Ts'o" <tytso@mit.edu>,
	Ext4 Developers List <linux-ext4@vger.kernel.org>
To: Mingming Cao <cmm@us.ibm.com>
In-Reply-To: <1312479840.7732.7.camel@mingming-laptop>
Sender: linux-ext4-owner@vger.kernel.org

On 08/04/2011 10:44 AM, Mingming Cao wrote:
> On Thu, 2011-08-04 at 11:25 -0400, Ted Ts'o wrote:
>> 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().
>>
>
> Yep, for the default 4k block size, if the offset is not block aligned,
> with the patch we could end of unnecessary unamp_page_range.
>
>> BTW, the name ext4_unmap_page_range() is a bit confusing; maybe we
>> should rename it to ext4_unmap_partial_page_buffers()?
>>
>
> The new name sounds better. It should only called for punch hole in the
> range (blocksize != pagesize) and (offset is block aligned) and (offset
> is not page aligned)
>
>> 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?
>>
>
> ext4_block_zero_page_range() also called from ext4 truncate code path,
> which only zero out within a block, but do not need to handle the
> partial page unmap. There are two logical steps need by punch hole, one
> is to zero out the non-block-aligned portion(like truncate), second is
> to unmap_partial_page_buffers(). It seems cleaner to separate the two
> logical steps out from the code simplify point of view.
>
> Regards,
> Mingming

Yeah looking at them again, I think I like the simpler v3.  V2 does both 
operations in one loop, but I think it's cleaner to keep them separate.

Allison Henderson

>> Regards,
>>
>> 						- Ted
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