linux-kernel - Re: [RFC] extending splice for copy offloading

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Date:	Wed, 2 Oct 2013 06:31:36 -0700 (PDT)
From:	David Lang <david@...g.hm>
To:	Jan Kara <jack@...e.cz>
cc:	Zach Brown <zab@...hat.com>, Miklos Szeredi <miklos@...redi.hu>,
	Ric Wheeler <rwheeler@...hat.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	Anna Schumaker <schumaker.anna@...il.com>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"Schumaker, Bryan" <Bryan.Schumaker@...app.com>,
	"Martin K. Petersen" <mkp@....net>, Jens Axboe <axboe@...nel.dk>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>,
	Eric Wong <normalperson@...t.net>
Subject: Re: [RFC] extending splice for copy offloading

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
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