reflink copy now works with nocow?

[Roman Mamedov <rm@romanrm.net>]

On Sat, 2 Nov 2019 08:49:37 -0500
Brian Hansen <dulanic@gmail.com> wrote:

> Hello,
> 
> First time i've sent to this group but I am trying to figure out the
> cause of this. Normal copy is working fine, but then if I use
> --reflink it says invalid argument. Not sure how to read some of this,
> but here is the strace.
> 
> I'm running kernel v4.15
> 
> Here is the full output of strace. I ran a strace on normal copy and
> most looks similar so I'm not able to figure out much here...
> 
> https://pastebin.com/raw/YmQ8FvCH

At first I was going to say, "oh it's because you are using 'chattr +C', or
mounted the filesystem as nocow, and reflink copying is prevented by those".
In fact this article from 2014 confirms that to be the case:
http://infotinks.com/btrfs-nodatacow-reflink-copies-snapshots/

But then I tested on my machine, and what used to fail, now works:

  # mkdir tmp
  # chattr +C tmp
  # echo abc > tmp/a
  # cp -a --reflink=always tmp/a tmp/b
  # lsattr tmp/
  ----------------C-- tmp/a
  ----------------C-- tmp/b

According to strace, the clone IOCTL succeeds:

...
  openat(AT_FDCWD, "tmp/b", O_WRONLY|O_CREAT|O_EXCL, 0600) = 4
  fstat(4, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
  ioctl(4, BTRFS_IOC_CLONE or FICLONE, 3) = 0
...

Same on kernels 4.14.151, 4.14.113 and 4.9.189.

So I wonder, is setting nocow via 'chattr +C' getting ignored now, or is there
an improvement that it no longer prevents reflink copying?

-- 
With respect,
Roman