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

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

On Wed, Oct 14, 2015 at 01:59:40PM -0400, Anna Schumaker wrote:
> On 10/12/2015 07:17 PM, Darrick J. Wong wrote:
> > 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.
> 
> So what I'm hearing is that I should drop the reflink and dedup flags and
> change this system call only perform a full copy (with preserving of
> sparseness), correct?  I can make those changes, but only if everybody is in
> agreement that it's the best way forward.

Sounds fine to me; I'll work on promoting EXTENT_SAME to the VFS.

> The only reason I haven't done anything to make this system call
> interruptible is because I haven't been able to find any documentation or
> examples for making system calls interruptible.  How do I do this?

I thought it was mostly a matter of sprinkling in "if (signal_pending(...))
return -ERESTARTSYS" type things whenever it's convenient to check.  The splice
code already seems to have this, though I'm no expert on what the splice code
actually does. :)

--D
> 
> Anna
> 
> > 
> > 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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