From: Miklos Szeredi Subject: Re: [PATCH 3/3] Add timeout feature Date: Wed, 09 Jul 2008 09:08:07 +0200 Message-ID: References: <20080630212450t-sato@mail.jp.nec.com> <20080701081026.GB16691@infradead.org> <20080707110730.GG5643@ucw.cz> <20080708231026.GP11558@disturbed> <20080708232031.GE18195@elf.ucw.cz> <20080709005254.GQ11558@disturbed> <20080709010922.GE9957@mit.edu> <20080709061621.GA5260@infradead.org> <20080708234120.5072111f@infradead.org> <20080708235502.1c52a586@infradead.org> Cc: miklos@szeredi.hu, miklos@szeredi.hu, hch@infradead.org, tytso@mit.edu, pavel@suse.cz, t-sato@yk.jp.nec.com, akpm@linux-foundation.org, viro@ZenIV.linux.org.uk, linux-ext4@vger.kernel.org, xfs@oss.sgi.com, dm-devel@redhat.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, axboe@kernel.dk, mtk.manpages@googlemail.com To: arjan@infradead.org Return-path: Received: from fxip-0047f.externet.hu ([88.209.222.127]:42314 "EHLO pomaz-ex.szeredi.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751801AbYGIHIc (ORCPT ); Wed, 9 Jul 2008 03:08:32 -0400 In-reply-to: <20080708235502.1c52a586@infradead.org> (message from Arjan van de Ven on Tue, 8 Jul 2008 23:55:02 -0700) Sender: linux-ext4-owner@vger.kernel.org List-ID: On Tue, 8 Jul 2008, Arjan van de Ven wrote: > I tihnk the idea there is > > freeze . do the snapshot op . unfreeze . make backup of snapshot Ah, so then my proposal would become run_frozen mountpoint do-snapshot do-backup release-snapshot and if they are afraid of deadlocks they can just implement the timeout in userspace: run_frozen -t timeout mountpoint do-snapshot 'run_frozen' can be a trivial 30 line app, that can be guaranteed not to deadlock. > one can argue about the need of doing the first 3 steps via a userland > loop; it sure sounds like one needs to be really careful to not do any > writes to the fs from the app that does snapshots (and that includes > doing any syscalls in the kernel that allocate memory.. just because > that already could cause unrelated data to be written from inside the > app. Not fun.) Userland always has to be careful when messing with raw devices. That alone is not a reason to put the snapshotting facility in kernel IMO. Miklos