Re: [PATCH v1 0/8] VFS: In-kernel copy system call

[Anna Schumaker]

On 09/09/2015 04:38 PM, Chris Mason wrote:
> On Wed, Sep 09, 2015 at 04:26:58PM -0400, Trond Myklebust wrote:
>> On Wed, Sep 9, 2015 at 4:09 PM, Chris Mason <address@hidden> wrote:
>>> On Tue, Sep 08, 2015 at 04:08:43PM -0700, Andy Lutomirski wrote:
>>>> On Tue, Sep 8, 2015 at 3:39 PM, Darrick J. Wong <address@hidden> wrote:
>>>>> On Tue, Sep 08, 2015 at 02:45:39PM -0700, Andy Lutomirski wrote:
>>>>>> What I meant by this was: if you ask for "regular copy", you may end
>>>>>> up with a reflink anyway.  Anyway, how can you reflink a range and
>>>>>> have the contents *not* be the same?
>>>>>
>>>>> reflink forcibly remaps fd_dest's range to fd_src's range.  If they didn't
>>>>> match before, they will afterwards.
>>>>>
>>>>> dedupe remaps fd_dest's range to fd_src's range only if they match, of 
>>>>> course.
>>>>>
>>>>> Perhaps I should have said "...if the contents are the same before the 
>>>>> call"?
>>>>>
>>>>
>>>> Oh, I see.
>>>>
>>>> Can we have a clean way to figure out whether two file ranges are the
>>>> same in a way that allows false negatives?  I.e. return 1 if the
>>>> ranges are reflinks of each other and 0 if not?  Pretty please?  I've
>>>> implemented that in the past on btrfs by syncing the ranges and then
>>>> comparing FIEMAP output, but that's hideous.
>>>
>>> I'd almost rather have a separate call, maybe unshare_file_range()?
>>>
>>
>> Doesn't it make more sense to put that functionality in fallocate()?
> 
> That works too, I'm just hoping to keep the copy_file_range stuff
> simple.

I agree with keeping copy_file_range() simple, especially for the initial 
merge.  Extra stuff can always be added in later :)

Anna

> 
> -chris
>