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From: Miklos Szeredi <email@example.com>
To: David Howells <firstname.lastname@example.org>
Cc: Al Viro <email@example.com>,
firstname.lastname@example.org, Christoph Hellwig <email@example.com>,
"Theodore Ts'o" <firstname.lastname@example.org>,
Andreas Dilger <email@example.com>,
"Darrick J. Wong" <firstname.lastname@example.org>,
Chris Mason <email@example.com>, Josef Bacik <firstname.lastname@example.org>,
Subject: Re: Making linkat() able to overwrite the target
Date: Tue, 14 Jan 2020 20:37:38 +0100 [thread overview]
Message-ID: <CAJfpegvbGrYb4wGDkuQXaeUGdeAMS0g7n6MezDV0BaLV_Bu1RQ@mail.gmail.com> (raw)
On Tue, Jan 14, 2020 at 7:06 PM David Howells <email@example.com> wrote:
> Al Viro <firstname.lastname@example.org> 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.
Create multiple sub-temp-dirs and use them alternatively.
I think there was a report for overlayfs with the same bottleneck
(copy up uses a temp dir, but now only for non-regular). Hasn't
gotten around to implementing this idea yet.
next prev parent reply other threads:[~2020-01-14 19:37 UTC|newest]
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
2020-01-14 19:37 ` Miklos Szeredi [this message]
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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