Re: [RFC PATCH v3 0/2] fs: add AT_REPLACE flag for linkat()

[Omar Sandoval <osandov@osandov.com>]

On Tue, Apr 24, 2018 at 08:26:10AM -0700, Omar Sandoval wrote:
> On Tue, Apr 24, 2018 at 06:21:07AM -0700, Christoph Hellwig wrote:
> > > Patch 1 implements the VFS support for this flag. The implementation
> > > resembles sys_renameat2(), and I took care to preserve all of the
> > > original error cases and make the new error cases consistent with
> > > rename.
> > 
> > Shouldn't we try to reuse the rename code and the ->rename method
> > instead of largely duplicating it?
> 
> As I mentioned in my cover letter, I did try, and it was uglier to reuse
> rename. 
> 
> For O_TMPFILE, we can't pass a meaningful old_dir to ->rename(), so now
> every ->rename() supporting AT_EMPTY_PATH has to grow a bunch of special
> cases for ignoring old_dir (I looked at Btrfs, ext4, and XFS, and it'd
> be messy for all three). Conversely, ->link() just needs the extra
> unlink before link logic (I added ->link2() for this RFC, but for the
> actual patch I'll just add the flag to ->link()).
> 
> On the VFS side, in my opinion, there isn't that much duplication -- we
> have the may_delete()/is_local_mountpoint() a la unlink, delegation
> handling, and some specific error cases we need to handle similarly to
> rename, but that's not too suprising for a new primitive.
> 
> > In fact I wonder if a better interface would just use renameat(2)
> > and accept the AT_EMPTY_PATH (for the source name only).
> 
> renameat() makes sense for the O_TMPFILE case, but linkat() + AT_REPLACE
> also supports a normal, named source file, and renameat() +
> AT_EMPTY_PATH for that doesn't make sense at all (rename this file by
> file descriptor, but don't really rename it?).

Oh, another argument in favor of linkat() is that existing programs
which are naming an O_TMPFILE will already be using linkat(), and it's
just a matter of adding the AT_REPLACE flag and skipping the temporary
name/rename dance.