Re: [RFC] extending splice for copy offloading

From: Miklos Szeredi
Date: Mon Sep 30 2013 - 11:24:34 EST

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.


- 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?

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