Re: [PATCH v1 0/8] VFS: In-kernel copy system call

[Andy Lutomirski <luto@amacapital.net>]

On Sep 13, 2015 4:25 PM, "Dave Chinner" <david@fromorbit.com> wrote:
>
> On Tue, Sep 08, 2015 at 04:08:43PM -0700, Andy Lutomirski wrote:
> > Can we have a clean way to figure out whether two file ranges are the
> > same in a way that allows false negatives?  I.e. return 1 if the
> > ranges are reflinks of each other and 0 if not?  Pretty please?  I've
> > implemented that in the past on btrfs by syncing the ranges and then
> > comparing FIEMAP output, but that's hideous.
>
> That fundamentally doesn't work for userspace, because the moment
> the filesystem drops it's locks on the inodes in the kernel after
> doing the comparison the mappings can change.  IOWs, by the time the
> information gets back to userspace, it's already wrong. e.g. cp made
> this mistake by trying to use FIEMAP to optimise hole detection in
> files and ended up with corrupt copies.
>
> It really doesn't matter what the syscall/ioctl interface is, trying
> to make application logic decisions based on inode block mappings
> from userspace is racy and not safe and will go wrong...
>

I agree, and that thing was just an experiment.  I'd love to see a
sane and correct interface, though.


--Andy