Re: [RFC] extending splice for copy offloading

Ric Wheeler
2013-09-30T15:42:29Z

On 09/30/2013 10:38 AM, Miklos Szeredi wrote:
> On Mon, Sep 30, 2013 at 4:28 PM, Ric Wheeler <rwheeler@redhat.com> wrote:
>> On 09/30/2013 10:24 AM, Miklos Szeredi wrote:
>>> On Mon, Sep 30, 2013 at 4:52 PM, Ric Wheeler <rwheeler@redhat.com> wrote:
>>>> On 09/30/2013 10:51 AM, Miklos Szeredi wrote:
>>>>> On Mon, Sep 30, 2013 at 4:34 PM, J. Bruce Fields <bfields@fieldses.org>
>>>>> wrote:
>>>>>>> My other worry is about interruptibility/restartability. Ideas?
>>>>>>>
>>>>>>> What happens on splice(from, to, 4G) and it's a non-reflink copy?
>>>>>>> Can the page cache copy be made restartable? Or should splice() be
>>>>>>> allowed to return a short count? What happens on (non-reflink) remote
>>>>>>> copies and huge request sizes?
>>>>>> If I were writing an application that required copies to be
>>>>>> restartable,
>>>>>> I'd probably use the largest possible range in the reflink case but
>>>>>> break the copy into smaller chunks in the splice case.
>>>>>>
>>>>> The app really doesn't want to care about that. And it doesn't want
>>>>> to care about restartability, etc.. It's something the *kernel* has
>>>>> to care about. You just can't have uninterruptible syscalls that
>>>>> sleep for a "long" time, otherwise first you'll just have annoyed
>>>>> users pressing ^C in vain; then, if the sleep is even longer, warnings
>>>>> about task sleeping too long.
>>>>>
>>>>> One idea is letting splice() return a short count, and so the app can
>>>>> safely issue SIZE_MAX requests and the kernel can decide if it can
>>>>> copy the whole file in one go or if it wants to do it in smaller
>>>>> chunks.
>>>>>
>>>> You cannot rely on a short count. That implies that an offloaded copy
>>>> starts
>>>> at byte 0 and the short count first bytes are all valid.
>>> Huh?
>>>
>>> - app calls splice(from, 0, to, 0, SIZE_MAX)
>>> 1) VFS calls ->direct_splice(from, 0, to, 0, SIZE_MAX)
>>> 1.a) fs reflinks the whole file in a jiffy and returns the size of
>>> the file
>>> 1 b) fs does copy offload of, say, 64MB and returns 64M
>>> 2) VFS does page copy of, say, 1MB and returns 1MB
>>> - app calls splice(from, X, to, X, SIZE_MAX) where X is the new offset
>>> ...
>>>
>>> The point is: the app is always doing the same (incrementing offset
>>> with the return value from splice) and the kernel can decide what is
>>> the best size it can service within a single uninterruptible syscall.
>>>
>>> Wouldn't that work?
>>>
>> No.
>>
>> Keep in mind that the offload operation in (1) might fail partially. The
>> target file (the copy) is allocated, the question is what ranges have valid
>> data.
> You are talking about case 1.a, right? So if the offload copy 0-64MB
> fails partially, we return failure from splice, yet some of the copy
> did succeed. Is that the problem? Why?
>
> Thanks,
> Miklos

The way the array based offload (and some software side reflink works) is not a
byte by byte copy. We cannot assume that a valid count can be returned or that
such a count would be an indication of a sequential segment of good data. The
whole thing would normally have to be reissued.

To make that a true assumption, you would have to mandate that in each of the
specifications (and sw targets)...

ric