Re: Making linkat() able to overwrite the target

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Jan 14, 2020 at 04:34:25PM +0000, David Howells wrote:
> With my rewrite of fscache and cachefiles:
> 
> 	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-iter
> 
> when a file gets invalidated by the server - and, under some circumstances,
> modified locally - I have the cache create a temporary file with vfs_tmpfile()
> that I'd like to just link into place over the old one - but I can't because
> vfs_link() doesn't allow you to do that.  Instead I have to either unlink the
> old one and then link the new one in or create it elsewhere and rename across.
> 
> Would it be possible to make linkat() take a flag, say AT_LINK_REPLACE, that
> causes the target to be replaced and not give EEXIST?  Or make it so that
> rename() can take a tmpfile as the source and replace the target with that.  I
> presume that, either way, this would require journal changes on ext4, xfs and
> btrfs.

Umm...  I don't like the idea of linkat() doing that - you suddenly get new
fun cases to think about (what should happen when the target is a mountpoint,
for starters?) _and_ you would have to add a magical flag to vfs_link() so
that it would know which tests to do.  As for rename...  How would that
work?  AT_EMPTY_PATH for source?  What happens if two threads do that
at the same time?  Should that case be always "create a new link, even
if you've got it by plain lookup somewhere"?  Worse, suppose you do that
to given tmpfile; what should happen to /proc/self/fd/* link to it?  Should
it point to new location, or...?