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Date: Tue, 14 Jan 2020 17:02:50 +0000 From: Al Viro <viro@...iv.linux.org.uk> To: David Howells <dhowells@...hat.com> Cc: linux-fsdevel@...r.kernel.org, hch@....de, tytso@....edu, adilger.kernel@...ger.ca, darrick.wong@...cle.com, clm@...com, josef@...icpanda.com, dsterba@...e.com, linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org, linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: Making linkat() able to overwrite the target 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...?
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