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...?
