From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f51.google.com (mail-pb0-f51.google.com [209.85.160.51]) by kanga.kvack.org (Postfix) with ESMTP id 44FB36B004D for ; Wed, 26 Feb 2014 18:09:56 -0500 (EST) Received: by mail-pb0-f51.google.com with SMTP id un15so1655196pbc.38 for ; Wed, 26 Feb 2014 15:09:55 -0800 (PST) Received: from mail-pb0-x231.google.com (mail-pb0-x231.google.com [2607:f8b0:400e:c01::231]) by mx.google.com with ESMTPS id gp2si2419112pac.157.2014.02.26.15.09.53 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 26 Feb 2014 15:09:55 -0800 (PST) Received: by mail-pb0-f49.google.com with SMTP id jt11so1646404pbb.22 for ; Wed, 26 Feb 2014 15:09:53 -0800 (PST) Date: Wed, 26 Feb 2014 15:08:58 -0800 (PST) From: Hugh Dickins Subject: Re: [PATCH v5 0/10] fs: Introduce new flag(FALLOC_FL_COLLAPSE_RANGE) for fallocate In-Reply-To: <20140226064224.GU13647@dastard> Message-ID: References: <1392741436-19995-1-git-send-email-linkinjeon@gmail.com> <20140224005710.GH4317@dastard> <20140225141601.358f6e3df2660d4af44da876@canb.auug.org.au> <20140225041346.GA29907@dastard> <20140226011347.GL13647@dastard> <20140226064224.GU13647@dastard> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Dave Chinner Cc: Hugh Dickins , Namjae Jeon , Andrew Morton , Matthew Wilcox , Theodore Ts'o , Stephen Rothwell , viro@zeniv.linux.org.uk, bpm@sgi.com, adilger.kernel@dilger.ca, jack@suse.cz, mtk.manpages@gmail.com, lczerner@redhat.com, linux-fsdevel@vger.kernel.org, xfs@oss.sgi.com, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Namjae Jeon On Wed, 26 Feb 2014, Dave Chinner wrote: > On Tue, Feb 25, 2014 at 08:45:15PM -0800, Hugh Dickins wrote: > > On Wed, 26 Feb 2014, Dave Chinner wrote: > > > On Tue, Feb 25, 2014 at 03:23:35PM -0800, Hugh Dickins wrote: > > > > > > > I should mention that when "we" implemented this thirty years ago, > > > > we had a strong conviction that the system call should be idempotent: > > > > that is, the len argument should indicate the final i_size, not the > > > > amount being removed from it. Now, I don't remember the grounds for > > > > that conviction: maybe it was just an idealistic preference for how > > > > to design a good system call. I can certainly see that defining it > > > > that way round would surprise many app programmers. Just mentioning > > > > this in case anyone on these lists sees a practical advantage to > > > > doing it that way instead. > > > > > > I don't see how specifying the end file size as an improvement. What > > > happens if you are collapse a range in a file that is still being > > > appended to by the application and so you race with a file size > > > update? IOWs, with such an API the range to be collapsed is > > > completely unpredictable, and IMO that's a fundamentally broken API. > > > > That's fine if you don't see the idempotent API as an improvement, > > I just wanted to put it on the table in case someone does see an > > advantage to it. But I think I'm missing something in your race > > example: I don't see a difference between the two APIs there. > > > Userspace can't sample the inode size via stat(2) and then use the value for a > syscall atomically. i.e. if you specify the offset you want to > collapse at, and the file size you want to have to define the region > to collapse, then the length you need to collapse is (current inode > size - end file size). If "current inode size" can change between > the stat(2) and fallocate() call (and it can), then the length being > collapsed is indeterminate.... Thanks for explaining more, I was just about to acknowledge what a good example that is. Indeed, it seems not unreasonable to be editing the earlier part of a file while the later part of it is still streaming in. But damn, it now occurs to me that there's still a problem at the streaming end: its file write offset won't be updated to reflect the collapse, so there would be a sparse hole at that end. And collapse returns -EPERM if IS_APPEND(inode). Never mind, I'm not campaigning for a change of interface anyway. Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org