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

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Sun, Oct 11, 2015 at 07:22:03AM -0700, Christoph Hellwig wrote:
> 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.

Hm.  Peng's patches only generalize the CLONE and CLONE_RANGE ioctls from
btrfs, however they don't port over the (vastly different) EXTENT_SAME ioctl.

What does everyone think about generalizing EXTENT_SAME?  The interface enables
one to ask the kernel to dedupe multiple file ranges in a single call.  That's
more complex than what I was proposing with COPY_FR_DEDUP(E), but I'm assuming
that the extra complexity buys us the ability to ... multi-dedupe at the same
time, with locks held on the source file?

I'm happy to generalize the existing EXTENT_SAME, but please yell if you really
hate the interface.

--D

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