linux-kernel - Re: New copyfile system call

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Date:	Sun, 31 Mar 2013 03:52:37 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	Andreas Dilger <adilger@...ger.ca>
CC:	Ric Wheeler <rwheeler@...hat.com>, Pavel Machek <pavel@....cz>,
	Jörn Engel <joern@...fs.org>,
	Andy Lutomirski <luto@...capital.net>,
	Zach Brown <zab@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Chris L. Mason" <clmason@...ionio.com>,
	Christoph Hellwig <hch@...radead.org>,
	Alexander Viro <aviro@...hat.com>,
	"Martin K. Petersen" <mkp@....net>,
	"Hannes Reinecke" <hare@...e.de>, Joel Becker <jlbec@...lplan.org>
Subject: Re: New copyfile system call - discuss before LSF?

On Sat, 2013-03-30 at 19:53 -0700, Andreas Dilger wrote:
> On 2013-03-30, at 16:21, Ric Wheeler <rwheeler@...hat.com> wrote:
> 
> > On 03/30/2013 05:57 PM, Myklebust, Trond wrote:
> >> On Mar 30, 2013, at 5:45 PM, Pavel Machek <pavel@....cz>
> >>  wrote:
> >> 
> >>> On Sat 2013-03-30 13:08:39, Andreas Dilger wrote:
> >>>> On 2013-03-30, at 12:49 PM, Pavel Machek wrote:
> >>>>> Hmm, really? AFAICT it would be simple to provide an
> >>>>> open_deleted_file("directory") syscall. You'd open_deleted_file(),
> >>>>> copy source file into it, then fsync(), then link it into filesystem.
> >>>>> 
> >>>>> That should have atomicity properties reflected.
> >>>> Actually, the open_deleted_file() syscall is quite useful for many
> >>>> different things all by itself.  Lots of applications need to create
> >>>> temporary files that are unlinked at application failure (without a
> >>>> race if app crashes after creating the file, but before unlinking).
> >>>> It also avoids exposing temporary files into the namespace if other
> >>>> applications are accessing the directory.
> >>> Hmm. open_deleted_file() will still need to get a directory... so it
> >>> will still need a path. Perhaps open("/foo/bar/mnt", O_DELETED) would
> >>> be acceptable interface?
> >>>                                    Pavel
> >> ...and what's the big plan to make this work on anything other than ext4 and btrfs?
> >> 
> >> Cheers,
> >>   Trond
> > 
> > I know that change can be a good thing, but are we really solving a pressing problem given that application developers have dealt with open/rename as the way to get "atomic" file creation for several decades now ?
> 
> Using open()+rename() has side effects:
> - changes ctime/mtime on parent directory
> - leaves temporary file in path during creation
> - leaves temporary file in namespace during operations, and after crash

So what is the actual problem that is being solved? Yes, the above may
be disadvantages, but none of them have proven to be show-stoppers so
far.

So far, I've seen no justification for Andy's atomicity requirement
other than "it would be nice if...". That's not enough IMO...


-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com
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