Re: [RFC PATCH] vfs: add permute operation

[Miklos Szeredi <miklos@szeredi.hu>]

On Thu, May 30, 2013 at 11:11 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, May 30, 2013 at 5:45 PM, Miklos Szeredi <miklos@szeredi.hu> wrote:

>>  1) check if destination directory is empty:  upper directory contains a
>> whiteout for each lower directory entry and nothing else
>>  2) if empty then remove whiteouts in destination directory
>>  3) and then go on with the normal rename procedure, replacing the empty
>> destination directory with the source directory ,
>>
>> This is done with directory locking, so atomicity is not usually a problem.
>> But in case of a crash between 2) and 3)  we just seriously corrupted the
>> overlay.
>>
>> Suggestions for fixing that?
>
> Why not just do the NFS thing. That has worked forever - using a
> sillyrename as a "pending deletion" instead of actually deleting
> things.
>
> So in between (1) and (2), silly-rename the pseudo-empty target. At
> that point (2) is no longer even an atomicity requirement, because you
> can do the whiteout removal later. In fact, you probably want to do it
> at the end, after doing the "real" rename.

Okay, nice idea.   More specifically we want to replace the directory
containing whiteouts with an opaque empty directory, which can be done
with a cross-rename.

Then we are left with basically two new variants of rename:

  - cross rename - exchange two names
  - plain overwriting rename but whiteout source

I'm fine with that.

As for userspace interfaces I think the cross-rename is useful enough
to justify a new syscall (rename/renameat don't have flags
unfortunately).

Thanks,
Miklos