Re: New copyfile system call - discuss before LSF?
From: Andy Lutomirski
Date: Tue Feb 26 2013 - 17:35:40 EST
On Tue, Feb 26, 2013 at 1:02 PM, Jörn Engel <joern@xxxxxxxxx> 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 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.
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