Re: [PATCH 1/1] [RFC] 64bit copy_file_range system call

[Andreas Dilger <adilger@dilger.ca>]

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> On Jun 15, 2017, at 7:07 AM, Olga Kornievskaia <aglo@umich.edu> wrote:
> 
> On Thu, Jun 15, 2017 at 4:15 AM, Christoph Hellwig <hch@infradead.org> wrote:
>> On Wed, Jun 14, 2017 at 01:06:53PM -0400, Olga Kornievskaia wrote:
>>> This is a proposal to allow 64bit count to copy and return as
>>> a result of a copy_file_range. No attempt was made to share code
>>> with the 32bit function because 32bit interface should probably
>>> get depreciated.
>>> 
>>> Why use 64bit? Current uses of 32bit are by clone_file_range()
>>> which could use 64bit count and NFS copy_file_range also supports
>>> 64bit count value.
>> 
>> Please provide a very good use case that actually matters to a large
>> portion of the Linux users.
> 
> Reason: it will not hurt any user but it will help for
> server-to-server NFS copy because for each invocation of of
> copy_file_range() it requires that the client sends COPY_NOTIFY, then
> COPY and then a copy triggers a establishment of clientid/session
> before the reading of the source file can happen. All that could be
> saved by have a 64bit value.

What is the overhead of sending a couple of RPCs vs. copying 4GB of
data on the server?

> Current copy_file_range would work for any file size, it would just
> need to be called in a loop by the application. With the given
> proposal, there wouldn't be a need for the application to loop due to
> the API size limitation. It might still loop if a partial copy was
> done.


Given that there always needs to be a loop to handle partial completion,
adding the 64-bit syscall doesn't really simplify the application at
all, but adds duplicate code to the kernel for little benefit.

Cheers, Andreas






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