Date: Tue, 1 Oct 2013 12:58:17 -0700
From: Zach Brown <zab@redhat.com>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: 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
Message-ID: <20131001195817.GE10831@lenny.home.zabbo.net>
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> - 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 :).

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! 

- z