how diff between hardlink trees works?

[Kai Meyer]

On 09/09/2011 12:39 PM, Vaibhav Jain wrote:
>
>
> On Fri, Sep 9, 2011 at 11:12 AM, Kai Meyer <kai at gnukai.com 
> <mailto:kai at gnukai.com>> wrote:
>
>     On 09/09/2011 09:05 AM, Vaibhav Jain wrote:
>>     Hi,
>>
>>     I am not able to understand how diff between two trees of which
>>     one is just contains hardlinks to another's files (cp -al )ing
>>     works.I am asking this question here because I need to build a
>>     custom kernel for which I need to generate patch. So the
>>     documentation suggests to create a hardlink copy of the kernel
>>     source tree using cp -al and then make changes to
>>     one of the trees and run a diff.I am wondering that if files are
>>     hardlinks then changes to one copy will affect another in which case
>>     diff should give no output.
>>     Also, the patch I created looks a little odd as it contains
>>     complete modified files instead of just the differences.
>>     Please help!
>>
>>     Thanks
>>     Vaibhav Jain
>>
>>
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>>     Kernelnewbies at kernelnewbies.org  <mailto:Kernelnewbies at kernelnewbies.org>
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>     Make the hard link copy like normal. Then delete the directory
>     that you are making changes to (in the hard link directory), then
>     copy the files over with out hard links. That way "most" of the
>     kernel tree is hard linked, and just the portion you want to work
>     on is a copy. That way the diff will work.
>
>     Otherwise, skip the hard link part all together, and just make a
>     full copy. Uses lots of disk space and takes longer to diff.
>
>     -Kai Meyer
>
>
>
> Hi Kai,
>
> Thanks for the reply. I need just one more favour.
> Could you please look at this document describing the procedure to build
> custom fedora kernel. It mentions the step to create hardlink to 
> generate but doesn't
> talk about deleting anything ?I just need to confirm if the article is 
> not accurate or if there is
> any error in my understanding.
> Whenever I follow it I get a patch that contains all of the content of 
> the changed files rather than just the changes.
>
> Here is the relevant portion :
>
>
>       Copy the Source Tree and Generate a Patch
>
> This step is for applying a patch to the kernel source. If a patch is 
> not needed, proceed to "Configure Kernel Options".
>
> Copy the source tree to preserve the original tree while making 
> changes to the copy:
>
> cp -r ~/rpmbuild/BUILD/kernel-2.6.$ver.$fedver/linux-2.6.$ver.$arch ~/rpmbuild/BUILD/kernel-2.6.$ver$fedver.orig
> cp -al ~/rpmbuild/BUILD/kernel-2.6.$ver.$fedver.orig ~/rpmbuild/BUILD/kernel-2.6.$ver.$fedver.new
>
> *The second |cp| command hardlinks the |.orig| and |.new| trees to 
> make |diff| run faster. Most text editors know how to break the 
> hardlink correctly to avoid problems.*
>
> Using vim on FC14, it treated the hard link as a hard link and thus 
> the above technique failed. It was necessary to repeat the original 
> copy used for the .orig directory for the .new directory. Note that 
> this uses twice the space.
>
> Make changes directly to the code in the |.new| source tree, or copy 
> in a modified file. This file might come from a developer who has 
> requested a test, from the upstream kernel sources, or from a 
> different distribution.
>
> After the |.new| source tree is modified, generate a patch. To 
> generate the patch, run |diff| against the entire |.new| and |.orig| 
> source trees with the following command:
>
> cd ~/rpmbuild/BUILD
> diff -uNrp kernel-2.6.$ver.$fedver.orig kernel-2.6.$ver.$fedver.new>  ../SOURCES/linux-2.6-my-new-patch.patch
>
> Thanks
> Vaibhav
>
>
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The article says this:

"Using vim on FC14, it treated the hard link as a hard link and thus the 
above technique failed. It was necessary to repeat the original copy 
used for the .orig directory for the .new directory. Note that this uses 
twice the space."

It means to say that some editors, like VIM, edit files in-place, and 
some files copy the original contents into some other buffer (memory or 
temporary file), and then effectively delete the file you're editing, 
and copy the modified file into place. The hard-link instructions are a 
"trick" to save time and space when you are modifying large code base, 
like the kernel. If your favorite editor is behaving like the observed 
behavor of VIM, then you will need to delete the hard link file, and put 
a regular copy of the file in place before making changes.

-Kai Meyer
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