Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755555Ab3I3Pm3 (ORCPT ); Mon, 30 Sep 2013 11:42:29 -0400 Received: from mx1.redhat.com ([209.132.183.28]:60134 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754486Ab3I3Pm1 (ORCPT ); Mon, 30 Sep 2013 11:42:27 -0400 Message-ID: <52498DB6.7060901@redhat.com> Date: Mon, 30 Sep 2013 09:41:58 -0500 From: Ric Wheeler User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130805 Thunderbird/17.0.8 MIME-Version: 1.0 To: Miklos Szeredi CC: "J. Bruce Fields" , "Myklebust, Trond" , Zach Brown , 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 References: <20130925210742.GG30372@lenny.home.zabbo.net> <20130926185508.GO30372@lenny.home.zabbo.net> <5244A68F.906@redhat.com> <20130927200550.GA22640@fieldses.org> <20130927205013.GZ30372@lenny.home.zabbo.net> <4FA345DA4F4AE44899BD2B03EEEC2FA9467EF2D7@SACEXCMBX04-PRD.hq.netapp.com> <52474839.2080201@redhat.com> <20130930143432.GG16579@fieldses.org> <52499026.3090802@redhat.com> <52498AA8.2090204@redhat.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3489 Lines: 77 On 09/30/2013 10:38 AM, Miklos Szeredi wrote: > On Mon, Sep 30, 2013 at 4:28 PM, Ric Wheeler wrote: >> On 09/30/2013 10:24 AM, Miklos Szeredi wrote: >>> On Mon, Sep 30, 2013 at 4:52 PM, Ric Wheeler wrote: >>>> On 09/30/2013 10:51 AM, Miklos Szeredi wrote: >>>>> On Mon, Sep 30, 2013 at 4:34 PM, J. Bruce Fields >>>>> wrote: >>>>>>> My other worry is about interruptibility/restartability. Ideas? >>>>>>> >>>>>>> What happens on splice(from, to, 4G) and it's a non-reflink copy? >>>>>>> Can the page cache copy be made restartable? Or should splice() be >>>>>>> allowed to return a short count? What happens on (non-reflink) remote >>>>>>> copies and huge request sizes? >>>>>> If I were writing an application that required copies to be >>>>>> restartable, >>>>>> I'd probably use the largest possible range in the reflink case but >>>>>> break the copy into smaller chunks in the splice case. >>>>>> >>>>> The app really doesn't want to care about that. And it doesn't want >>>>> to care about restartability, etc.. It's something the *kernel* has >>>>> to care about. You just can't have uninterruptible syscalls that >>>>> sleep for a "long" time, otherwise first you'll just have annoyed >>>>> users pressing ^C in vain; then, if the sleep is even longer, warnings >>>>> about task sleeping too long. >>>>> >>>>> One idea is letting splice() return a short count, and so the app can >>>>> safely issue SIZE_MAX requests and the kernel can decide if it can >>>>> copy the whole file in one go or if it wants to do it in smaller >>>>> chunks. >>>>> >>>> You cannot rely on a short count. That implies that an offloaded copy >>>> starts >>>> at byte 0 and the short count first bytes are all valid. >>> Huh? >>> >>> - 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 >>> ... >>> >>> 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? >>> >> No. >> >> Keep in mind that the offload operation in (1) might fail partially. The >> target file (the copy) is allocated, the question is what ranges have valid >> data. > You are talking about case 1.a, right? So if the offload copy 0-64MB > fails partially, we return failure from splice, yet some of the copy > did succeed. Is that the problem? Why? > > Thanks, > Miklos The way the array based offload (and some software side reflink works) is not a byte by byte copy. We cannot assume that a valid count can be returned or that such a count would be an indication of a sequential segment of good data. The whole thing would normally have to be reissued. To make that a true assumption, you would have to mandate that in each of the specifications (and sw targets)... ric -- 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/