Re: [PATCH v1 2/3] VFS permit cross device vfs_copy_file_range

[Olga Kornievskaia <kolga@netapp.com>]

> On Mar 2, 2017, at 11:38 AM, Olga Kornievskaia <kolga@netapp.com> wrote:
> 
> 
>> On Mar 2, 2017, at 11:07 AM, Christoph Hellwig <hch@lst.de> wrote:
>> 
>> On Thu, Mar 02, 2017 at 11:02:10AM -0500, Olga Kornievskaia wrote:
>>> Allow nfs_copy_file_range to copy across devices.
>>> NFSv4.2 inter server to server copy always copies across devices, and
>>> NFSv4.2 intra server to server copy can copy across devices on the same
>>> server.
>>> 
>>> If a file system's fileoperations copy_file_range operation prohibits
>>> cross-device copies, fall back to do_splice_direct. This is needed for
>>> nfsd_copy_file_range() which is called by the inter server to server
>>> destination server acting as an NFS client, and reading the file from
>>> the source server.
>> 
>> NAK, we really should not do operations between different superblocks.
> 
> Can you provide some reasoning as to why? What would it break? The reasoning for including one is to allow for a file system to achieve better performance which seems like a feature that would be of great benefit.

Christoph, could you please elaborate on your objection.

Al, could you weigh in with regards to if and what would it take to allow for copy_file_range() to allow for copies between different superblocks.

We would appreciate the feedback of how can we enable this performance feature to be useful to users.

Thank you.