Re: [RFC] extending splice for copy offloading

From: Miklos Szeredi
Date: Mon Sep 30 2013 - 11:38:40 EST

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?

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