Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754336Ab3JBM6p (ORCPT ); Wed, 2 Oct 2013 08:58:45 -0400 Received: from cantor2.suse.de ([195.135.220.15]:45782 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753655Ab3JBM6l (ORCPT ); Wed, 2 Oct 2013 08:58:41 -0400 Date: Wed, 2 Oct 2013 14:58:37 +0200 From: Jan Kara To: Zach Brown Cc: Miklos Szeredi , Ric Wheeler , "J. Bruce Fields" , "Myklebust, Trond" , Anna Schumaker , Kernel Mailing List , Linux-Fsdevel , "linux-nfs@vger.kernel.org" , "Schumaker, Bryan" , "Martin K. Petersen" , Jens Axboe , Mark Fasheh , Joel Becker , Eric Wong Subject: Re: [RFC] extending splice for copy offloading Message-ID: <20131002125837.GA32181@quack.suse.cz> References: <20130927205013.GZ30372@lenny.home.zabbo.net> <4FA345DA4F4AE44899BD2B03EEEC2FA9467EF2D7@SACEXCMBX04-PRD.hq.netapp.com> <52474839.2080201@redhat.com> <20130930143432.GG16579@fieldses.org> <52499026.3090802@redhat.com> <20131001195817.GE10831@lenny.home.zabbo.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131001195817.GE10831@lenny.home.zabbo.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1813 Lines: 41 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 Honza -- Jan Kara SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/