Re: [RFC] extending splice for copy offloading

[Ric Wheeler <rwheeler@redhat.com>]

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?
>
> Thanks,
> Miklos

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.

I don't see that (2) is interesting or really needed to be done in the kernel. 
If nothing else, it tends to confuse the discussion....

ric