Return-Path: linux-nfs-owner@vger.kernel.org Received: from mail-qc0-f181.google.com ([209.85.216.181]:51267 "EHLO mail-qc0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755883Ab3I3PY1 (ORCPT ); Mon, 30 Sep 2013 11:24:27 -0400 Received: by mail-qc0-f181.google.com with SMTP id q4so3746121qcx.12 for ; Mon, 30 Sep 2013 08:24:26 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <52499026.3090802@redhat.com> 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> Date: Mon, 30 Sep 2013 17:24:26 +0200 Message-ID: Subject: Re: [RFC] extending splice for copy offloading From: Miklos Szeredi To: Ric Wheeler 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 Content-Type: text/plain; charset=UTF-8 Sender: linux-nfs-owner@vger.kernel.org List-ID: 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? Thanks, Miklos