linux-ext4 - Making linkat() able to overwrite the target

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Date:   Tue, 14 Jan 2020 16:34:25 +0000
From:   David Howells <dhowells@...hat.com>
To:     linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk, hch@....de,
        tytso@....edu, adilger.kernel@...ger.ca, darrick.wong@...cle.com,
        clm@...com, josef@...icpanda.com, dsterba@...e.com
cc:     dhowells@...hat.com, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Making linkat() able to overwrite the target

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.

Thanks,
David