linux-kernel - Re: New copyfile system call

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Date:	Sat, 30 Mar 2013 19:53:06 -0700
From:	Andreas Dilger <adilger@...ger.ca>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	"Myklebust, Trond" <Trond.Myklebust@...app.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 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

Cheers, Andreas--
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