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Subject: Re: [ckrm-tech] [PATCH 0/5] Allow more than PAGESIZE data read in
	configfs
From: Matt Helsley <matthltc@us.ibm.com>
To: Greg KH <greg@kroah.com>
Cc: Chandra Seetharaman <sekharan@us.ibm.com>, Paul Menage <menage@google.com>,
       linux-kernel@vger.kernel.org, ckrm-tech@lists.sourceforge.net
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Organization: IBM Linux Technology Center
Date: Sat, 14 Oct 2006 16:14:14 -0700
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On Fri, 2006-10-13 at 23:17 -0700, Greg KH wrote:
> On Fri, Oct 13, 2006 at 04:40:08PM -0700, Matt Helsley wrote:
> > On Thu, 2006-10-12 at 16:51 -0700, Greg KH wrote:
> > 
> > <snip>
> > 
> > > > BTW, it it not just CKRM/RG, Paul Menage as recently extracted the
> > > > processes aggregation from cpuset to have an independent infrastructure
> > > > (http://marc.theaimsgroup.com/?l=ckrm-tech&m=116006307018720&w=2), which
> > > > has its own file system. I was advocating him to use configfs. But, he
> > > > also has this issue/limitation. 
> > > 
> > > That's one reason it is so easy to just write your own filesystem then.
> > > What is it these days, less than 200 lines of code?  I bet you can even
> > 
> > For my_school_project_fs perhaps 200 lines is sufficient.
> > 
> > Paul Menage's patch which Chandra was referring to:
> > 
> > http://lkml.org/lkml/2006/9/28/104
> > 
> > is 1700 insertions. RCFS was around 1500 lines -- similar to Paul's
> > patch -- before we moved to configfs and reduced that to about 300-400
> > lines. This suggests we'd need around 1500 lines of filesystem code --
> > 7.5 times your estimate.
> 
> Then I suggest that you are doing something extremely wrong here.  The
> base filesystem code for both debugfs and securityfs, is around 200
> lines of code, including comments.  And they are both not
> "my_school_project_fs".
> 
> If you want to get a bit fancier, and parse some mount options and
> provide a persistant mount state, like usbfs, it grows to about 350
> lines of code.
> 
> Again, not a "fake" filesystem by any means.
> 
> And, for another example, ndevfs was posted to lkml over a year ago as a
> bad joke and can be found at:
> 	http://www.kernel.org/pub/linux/kernel/people/gregkh/gregkh-2.6/patches/bad/ndevfs.patch
> and weighs in at a whopping 249 lines for all of inode.c (comments and
> whitespace included.)  It was a full-blown filesystem that handled
> almost exactly what you will need to handle.
> 
> So please, before you critise me for not knowing exactly how much work
> it takes to implement a ram-based filesystem for something like this,
> take a look at a few of the already published and shipping
> implementations, and note who wrote them...
> 
> Otherwise you look quite foolish.
> 
> > > condence more things to make it 100 lines if you really try.  That seems
> > > much more sane than trying to bend configfs into something different.
> > 
> > 	I don't agree. I think it's insane not to use configfs just because we
> > need a list of pids for each group of tasks.
> 
> Perhaps because your usage is not what it is intended for?  Yeah, I
> know, if all you have is a hammer...
> 
> > > Why are people so opposed to creating their own filesystems?
> > 
> > 	There are lots of reasons not to create your own filesystem. When
> > you're not already a kernel maintainer it's no small task to create and
> > get a non-trivial filesystem accepted into the kernel. Getting people to
> > review whole new filesystems has its own problems:
> > 
> > http://www.ussg.iu.edu/hypermail/linux/kernel/0610.1/1928.html
> 
> There's a world of difference between something as complex as unionfs
> and something that merely makes a few calls to the libfs code already in
> the kernel.
> 
> And if you don't realize this, then yes, I would not recommend that you
> should write your own fs.  But I would then recommend that the task then
> be passed on to someone else in your group who does have the capability
> to do so...
> 
> greg k-h

	You're right. I'm sorry I suggested that 200 lines might be too small
an estimate. Clearly that was a moronic suggestion on my part and I'm
sorry you took it so personally.

	-Matt Helsley

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