Re: [RFC] extending splice for copy offloading

David Lang
2013-10-02T13:32:52Z

On Wed, 2 Oct 2013, Jan Kara wrote:

> On Tue 01-10-13 12:58:17, Zach Brown wrote:
>>> - 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
>>
>> (It's not SIZE_MAX. It's MAX_RW_COUNT. INT_MAX with some
>> PAGE_CACHE_SIZE rounding noise. For fear of weird corners of fs code
>> paths that still use int, one assumes.)
>>
>>> 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?
>>
>> It seems like it should, if people are willing to allow splice() to
>> return partial counts. Quite a lot of IO syscalls technically do return
>> partial counts today if you try to write > MAX_RW_COUNT :).
> Yes. Also POSIX says that application must handle such case for read &
> write. But in practice programmers are lazy.
>
>> But returning partial counts on the order of a handful of megs that the
>> file systems make up as the point of diminishing returns is another
>> thing entirely. I can imagine people being anxious about that.
>>
>> I guess we'll find out!
> Return 4 KB once in a while to screw up buggy applications from the
> start :-p

or at least have a debugging option early on that does this so people can use it
to find such buggy apps.

David Lang