Re: [RFC] extending splice for copy offloading

[Miklos Szeredi]

On Mon, Sep 30, 2013 at 4:28 PM, Ric Wheeler <rwheeler@xxxxxxxxxx> wrote:
> On 09/30/2013 10:24 AM, Miklos Szeredi wrote:
>>
>> On Mon, Sep 30, 2013 at 4:52 PM, Ric Wheeler <rwheeler@xxxxxxxxxx> wrote:
>>>
>>> On 09/30/2013 10:51 AM, Miklos Szeredi wrote:
>>>>
>>>> On Mon, Sep 30, 2013 at 4:34 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx>
>>>> 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
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