Re: [RFC] extending splice for copy offloading

[Miklos Szeredi <miklos@szeredi.hu>]

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?

Thanks,
Miklos