Re: [PATCH v5 8/9] vfs: Add vfs_copy_file_range() support for pagecache copies

[Christoph Hellwig <hch@infradead.org>]

On Wed, Sep 30, 2015 at 01:26:52PM -0400, Anna Schumaker wrote:
> This allows us to have an in-kernel copy mechanism that avoids frequent
> switches between kernel and user space.  This is especially useful so
> NFSD can support server-side copies.
> 
> I make pagecache copies configurable by adding three new (exclusive)
> flags:
> - COPY_FR_REFLINK tells vfs_copy_file_range() to only create a reflink.
> - COPY_FR_COPY does a full data copy, but may be filesystem accelerated.
> - COPY_FR_DEDUP creates a reflink, but only if the contents of both
>   ranges are identical.

All but FR_COPY really should be a separate system call.  Clones (an
dedup as a special case of clones) are really a separate beast from file
copies.

If I want to clone a file I either want it clone fully or fail, not copy
a certain amount.  That means that a) we need to return an error not
short "write", and b) locking impementations are important - we need to
prevent other applications from racing with our clone even if it is
large, while to get these semantics for the possible short returning
file copy will require a proper userland locking protocol. Last but not
least file copies need to be interruptible while clones should be not.
All this is already important for local file systems and even more
important for NFS exporting.

So I'd suggest to drop this patch and just let your syscall handle
actualy copies with all their horrors.  We can go with Peng's patches
to generalize the btrfs ioctls for clones for now which is what everyone
already uses anyway, and then add a separate sys_file_clone later.