Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754404Ab3JBNcw (ORCPT ); Wed, 2 Oct 2013 09:32:52 -0400 Received: from mail.lang.hm ([64.81.33.126]:43652 "EHLO bifrost.lang.hm" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753559Ab3JBNcs (ORCPT ); Wed, 2 Oct 2013 09:32:48 -0400 Date: Wed, 2 Oct 2013 06:31:36 -0700 (PDT) From: David Lang X-X-Sender: dlang@asgard.lang.hm To: Jan Kara cc: Zach Brown , 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 In-Reply-To: <20131002125837.GA32181@quack.suse.cz> Message-ID: 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> <20131002125837.GA32181@quack.suse.cz> User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1930 Lines: 43 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 -- 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/