[Ocfs2-devel] [PATCH 1/3] fs: Document the reflink(2) system call.

[Theodore Tso]

On Mon, May 04, 2009 at 02:18:54PM -0700, Joel Becker wrote:
> 	More thinking.  It looks like we'll restrict reflink() to owners
> or people with CAP_FCHOWN.  This prevents some quota DoS behavior.
> 	We need to pre-charge all quota.  That means a reflink must be
> charged the entire size of the file.  So, if I do:
> 
>   # dd if=/dev/zero bs=1M count=1 of=foo
>   # reflink foo bar
> 
> I am now charged 2MB of quota, even though foo and bar share the same
> 1MB of space.

Yep; but as long as you do this, why do you need CAP_FCHOWN?  

Suppose Alice has a 1MB file, and Bob creates a reflink to it.  The
reflink would be owned by Bob, and Bob would be charged the 1MB quota.
This mirrors exactly what happens if Bob were to make a copy of the
file, and we want to make the creation of reflink mirror a copy, right?

In that case, as long as Bob has read access to the file, he should be
allowed to create a reflink.

That way when you do the copy-on-write, Bob will continue to be
charged the 1MB quota, which is what you want.  So pre-charging the
quota makes the most amount of sense.

	    	       	     	     - Ted