Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S968132Ab3E3KPo (ORCPT ); Thu, 30 May 2013 06:15:44 -0400 Received: from mail-qa0-f54.google.com ([209.85.216.54]:46546 "EHLO mail-qa0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S967701Ab3E3KPj (ORCPT ); Thu, 30 May 2013 06:15:39 -0400 MIME-Version: 1.0 X-Originating-IP: [188.6.195.195] In-Reply-To: References: <20130527153313.GC1842@tucsk.piliscsaba.szeredi.hu> Date: Thu, 30 May 2013 12:15:38 +0200 Message-ID: Subject: Re: [RFC PATCH] vfs: add permute operation From: Miklos Szeredi To: Linus Torvalds Cc: David Howells , Christoph Hellwig , Andrew Morton , Linux-Fsdevel , Al Viro , Kernel Mailing List , Zach Brown Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1849 Lines: 47 On Thu, May 30, 2013 at 11:11 AM, Linus Torvalds wrote: > On Thu, May 30, 2013 at 5:45 PM, Miklos Szeredi wrote: >> 1) check if destination directory is empty: upper directory contains a >> whiteout for each lower directory entry and nothing else >> 2) if empty then remove whiteouts in destination directory >> 3) and then go on with the normal rename procedure, replacing the empty >> destination directory with the source directory , >> >> This is done with directory locking, so atomicity is not usually a problem. >> But in case of a crash between 2) and 3) we just seriously corrupted the >> overlay. >> >> Suggestions for fixing that? > > Why not just do the NFS thing. That has worked forever - using a > sillyrename as a "pending deletion" instead of actually deleting > things. > > So in between (1) and (2), silly-rename the pseudo-empty target. At > that point (2) is no longer even an atomicity requirement, because you > can do the whiteout removal later. In fact, you probably want to do it > at the end, after doing the "real" rename. Okay, nice idea. More specifically we want to replace the directory containing whiteouts with an opaque empty directory, which can be done with a cross-rename. Then we are left with basically two new variants of rename: - cross rename - exchange two names - plain overwriting rename but whiteout source I'm fine with that. As for userspace interfaces I think the cross-rename is useful enough to justify a new syscall (rename/renameat don't have flags unfortunately). Thanks, Miklos -- 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/