Date: Wed, 2 Oct 2013 06:31:36 -0700 (PDT)
From: David Lang <david@lang.hm>
To: Jan Kara <jack@suse.cz>
cc: Zach Brown <zab@redhat.com>, Miklos Szeredi <miklos@szeredi.hu>,
        Ric Wheeler <rwheeler@redhat.com>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        "Myklebust, Trond" <Trond.Myklebust@netapp.com>,
        Anna Schumaker <schumaker.anna@gmail.com>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-Fsdevel <linux-fsdevel@vger.kernel.org>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
        "Schumaker, Bryan" <Bryan.Schumaker@netapp.com>,
        "Martin K. Petersen" <mkp@mkp.net>, Jens Axboe <axboe@kernel.dk>,
        Mark Fasheh <mfasheh@suse.com>, Joel Becker <jlbec@evilplan.org>,
        Eric Wong <normalperson@yhbt.net>
Subject: Re: [RFC] extending splice for copy offloading
In-Reply-To: <20131002125837.GA32181@quack.suse.cz>
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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