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From: David Howells <dhowells@redhat.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: dhowells@redhat.com, linux-fsdevel@vger.kernel.org, hch@lst.de,
	tytso@mit.edu, adilger.kernel@dilger.ca, darrick.wong@oracle.com,
	clm@fb.com, josef@toxicpanda.com, dsterba@suse.com,
	linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: Making linkat() able to overwrite the target
Date: Tue, 14 Jan 2020 18:06:10 +0000
Message-ID: <9351.1579025170@warthog.procyon.org.uk> (raw)
In-Reply-To: <20200114170250.GA8904@ZenIV.linux.org.uk>

Al Viro <viro@zeniv.linux.org.uk> wrote:

> > 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?

Don't allow it onto directories, S_AUTOMOUNT-marked inodes or anything that's
got something mounted on it.

> ) _and_ you would have to add a magical flag to vfs_link() so
> that it would know which tests to do.

Yes, I suggested AT_LINK_REPLACE as said magical flag.

> As for rename...

Yeah - with further thought, rename() doesn't really work as an interface,
particularly if a link has already been made.

Do you have an alternative suggestion?  There are two things I want to avoid:

 (1) Doing unlink-link or unlink-create as that leaves a window where the
     cache file is absent.

 (2) Creating replacement files in a temporary directory and renaming from
     there over the top of the target file as the temp dir would then be a
     bottleneck that spends a lot of time locked for creations and renames.

David


  parent reply index

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-14 16:34 David Howells
2020-01-14 17:02 ` Al Viro
2020-01-14 18:06 ` David Howells [this message]
2020-01-14 19:37   ` Miklos Szeredi
2020-01-17  0:46   ` Colin Walters
2020-01-17  9:57     ` Amir Goldstein
2020-01-17 11:42   ` David Howells
2020-01-17 16:22     ` Omar Sandoval
2020-01-17 16:39     ` David Howells
2020-01-15  8:36 ` Christoph Hellwig

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