linux-kernel - Re: New copyfile system call

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Date:	Tue, 26 Feb 2013 14:35:14 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Jörn Engel <joern@...fs.org>
Cc:	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 Tue, Feb 26, 2013 at 1:02 PM, Jörn Engel <joern@...fs.org> wrote:
> On Mon, 25 February 2013 13:14:52 -0800, Andy Lutomirski wrote:
>>
>> 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,...
>
> If you take a filesystem like ext[34] you are out of luck.  In those
> filesystems it may not even be theoretically possible to get the
> cleanup right for pathological cases.  And if you ignore pathological
> cases and depend on userspace to do the cleanup for you, you have to
> do ABI extentions that I don't want to mention with Al on Cc:.  My
> personal notebook ran such a kernel for several years until hardware
> improved to a point that I no longer wanted to forward-port the
> patches.  It worked but it was far from pretty.
>
> If you have a filesystem where you can simply bumb a reference count
> to copy the file content, implementation is fairly straightforward.
> But having a system call that is effectively limited to btrfs means
> pretty much noone will use it - beside the people looking for
> potential kernel exploits.

:)

>
> So my vote clearly goes to some variant of sendfile or splice.

Don't get me wrong -- the vpsendfile (or whatever it's called) idea
sounds extremely useful too.

--Andy
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