From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id A17B3C636D4 for ; Fri, 10 Feb 2023 19:43:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233508AbjBJTny (ORCPT ); Fri, 10 Feb 2023 14:43:54 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53340 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233488AbjBJTnp (ORCPT ); Fri, 10 Feb 2023 14:43:45 -0500 Received: from hr2.samba.org (hr2.samba.org [IPv6:2a01:4f8:192:486::2:0]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 877D43770C; Fri, 10 Feb 2023 11:42:53 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=samba.org; s=42; h=From:Cc:To:Date:Message-ID; bh=CbB3d2PjMe4L8HXRGPzj+z4SVKmNX8v3M7gr2hTK0UQ=; b=cyijFghPs/6fu47ULVexryV74P XVSei/aSlTama2pxp80JLpfs/yMxuGC/l/3WpPKpfcM2cSPP2ond3dKQjM8QzxWgRWXErQ2UhfRcu DawxeXk9+wofZQ5eFkq/kcYlxOFurTfmV/97Bl+tFSNsSFbBIBZgtaHqJ/LWsbgZXcJkK9jwQpUZR 0xvYUv0BjPgEJHsqo4/Ruj9AotCY9y0K7TB9HVe31thCtHHe2wZ3dWLQPEi7qfSfuLg6C1YQuRpZo xRJAuJ85JCElNv3kZczMSFzH/vKGjpfwLpiW7eP7FJARJ+r0p+RW/QdGT8p/OWkyqbcj+VOqOfO6o jVR/YtzQWqEdipqO0Va30ex/ZH0K+CKb50ky6s6B20ZbmtTTtvRgggjMGboFuEbzhrVvtcX5y64dN uiUKFAI0JOkuGtuh4+/BObBF5dE6X/p0bQWgwuYqYIuNPtHOV6X1gLRPDjOYEvsRziR87/l4vSqPO 1L4vT94nl9IbdcNZeIjCuKvo; Received: from [127.0.0.2] (localhost [127.0.0.1]) by hr2.samba.org with esmtpsa (TLS1.3:ECDHE_SECP256R1__ECDSA_SECP256R1_SHA256__CHACHA20_POLY1305:256) (Exim) id 1pQZHZ-00D3Ui-V3; Fri, 10 Feb 2023 19:42:06 +0000 Message-ID: <58e7e5d1-c231-a9cf-6d72-47a96c4285b3@samba.org> Date: Fri, 10 Feb 2023 20:42:05 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1 Subject: Re: copy on write for splice() from file to pipe? Content-Language: en-US To: Jeremy Allison , Linus Torvalds Cc: Andy Lutomirski , Jens Axboe , Linux API Mailing List , Dave Chinner , "linux-kernel@vger.kernel.org" , Matthew Wilcox , Al Viro , linux-fsdevel , Samba Technical , io-uring References: <20230210021603.GA2825702@dread.disaster.area> <20230210040626.GB2825702@dread.disaster.area> <20230210065747.GD2825702@dread.disaster.area> From: Stefan Metzmacher In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org Am 10.02.23 um 20:27 schrieb Jeremy Allison: > On Fri, Feb 10, 2023 at 11:18:05AM -0800, Linus Torvalds via samba-technical wrote: >> >> We should point the fingers at either the _user_ of splice - as Jeremy >> Allison has done a couple of times - or we should point it at the sink >> that cannot deal with unstable sources. >> .... >> - it sounds like the particular user in question (samba) already very >> much has a reasonable model for "I have exclusive access to this" that >> just wasn't used > > Having said that, I just had a phone discussion with Ralph Boehme > on the Samba Team, who has been following along with this in > read-only mode, and he did point out one case I had missed. > > 1). Client opens file with a lease. Hurrah, we think we can use splice() ! > 2). Client writes into file. > 3). Client calls SMB_FLUSH to ensure data is on disk. > 4). Client reads the data just wrtten to ensure it's good. > 5). Client overwrites the previously written data. > > Now when client issues (4), the read request, if we > zero-copy using splice() - I don't think theres a way > we get notified when the data has finally left the > system and the mapped splice memory in the buffer cache > is safe to overwrite by the write (5). > > So the read in (4) could potentially return the data > written in (5), if the buffer cache mapped memory has > not yet been sent out over the network. > > That is certainly unexpected behavior for the client, > even if the client leased the file. > > If that's the case, then splice() is unusable for > Samba even in the leased file case. I think we just need some coordination in userspace. What might be helpful in addition would be some kind of notification that all pages are no longer used by the network layer, IORING_OP_SENDMSG_ZC already supports such a notification, maybe we can build something similar. >>   Maybe this thread raised some awareness of it for some people, but >> more realistically - maybe we can really document this whole issue >> somewhere much more clearly > > Complete comprehensive documentation on this would > be extremely helpful (to say the least :-). Yes, good documentation is always good :-)