Re: [PATCH v1 2/3] NFSD: Implement the COPY call

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, Dec 04, 2015 at 12:05:05PM -0500, Anna Schumaker wrote:
> On 12/04/2015 11:49 AM, J. Bruce Fields wrote:
> > On Fri, Dec 04, 2015 at 10:49:24AM -0500, Anna Schumaker wrote:
> >> On 12/04/2015 10:45 AM, J. Bruce Fields wrote:
> >>> On Thu, Dec 03, 2015 at 03:55:35PM -0500, Anna Schumaker wrote:
> >>>> @@ -498,6 +499,22 @@ __be32 nfsd4_set_nfs4_label(struct svc_rqst *rqstp, struct svc_fh *fhp,
> >>>>  }
> >>>>  #endif
> >>>>  
> >>>> +ssize_t nfsd_copy_range(struct file *src, u64 src_pos,
> >>>> +		       struct file *dst, u64 dst_pos,
> >>>> +		       u64 count)
> >>>> +{
> >>>> +	ssize_t bytes;
> >>>> +	u64 limit = 0x10000000;
> >>>
> >>> Why that value?  Could I get a comment here?
> >>
> >> Whoops!  I had a comment there at one point, but I must have deleted it :(.  That value is to cap copies to 256MB.
> > 
> > Could you include some justification for the choice of that particular
> > value?
> 
> Yeah, I can run tests with different values and include the results in v2.
> 
> > 
> >>>> +	if (count > limit)
> >>>> +		count = limit;
> >>>> +
> >>>> +	bytes = vfs_copy_file_range(src, src_pos, dst, dst_pos, count, 0);
> >>>
> >>> Sorry, I lost track of the copy discussions: does this only work on
> >>> filesystems with special support, or does it fall back on doing the copy
> >>> by hand?  Which filesystems (of the exportable filesystems) support
> >>> this?
> >>
> >> The system call falls back on doing the copy by hand if there is no filesystem acceleration.
> > 
> > Is this practical?  It means a huge range in possible latency of the
> > single COPY call depending on filesystem.
> 
> That's why I'm breaking copies into smaller chunks, rather than doing everything at once.
> 
> > 
> > I guess I can live with it and we can see if people run into problems in
> > practice.  But let's make sure this is documented.
> 
> Okay.  I'll add documentation about this!

OK, thanks.  I think those are my only concerns, good to see that this
is all the new code we need now.--b.