linux-kernel - Re: New copyfile system call

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Date:	Sat, 30 Mar 2013 15:40:55 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Pavel Machek <pavel@....cz>
Cc:	Jörn Engel <joern@...fs.org>,
	Zach Brown <zab@...hat.com>,
	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Ric Wheeler <rwheeler@...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, Mar 30, 2013 at 12:49 PM, Pavel Machek <pavel@....cz> wrote:
> Hi!
>
>> > I thought the first thing people would ask for is to atomically create a
>> > new file and copy the old file into it (at least on local file systems).
>> >  The idea is that nothing should see an empty destination file, either
>> > by race or by crash.  (This feature would perhaps be described as a
>> > pony, but it should be implementable.)
>>
>> Having already wasted many week trying to implement your pony, I would
>> consider it about as possible as winning the lottery three times in a
>> row.  It clearly is in theory and yet,...
>
> Hmm, really? AFAICT it would be simple to provide open_deleted_file("directory")
> syscall. You'd open_deleted_file(), copy source file into it, then
> fsync(), then link it into filesystem.

Isn't linking a deleted file back into the filesystem explicitly
forbidden?  I'm pretty sure that linking from /proc/fd/whatever
doesn't work.  (I've often wanted a flink system call that takes a
file descriptor and links it somewhere.  If it came with an option to
control whether it would overwrite an existing file, even better.)

--Andy
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