From: "Takashi Sato" Subject: Re: [PATCH 3/3] Add timeout feature Date: Thu, 9 Oct 2008 19:12:17 +0900 Message-ID: <0AD458D29A8E4938BB3914BB5C562C39@nsl.ad.nec.co.jp> References: <20080908205337t-sato@mail.jp.nec.com> <20080908171119.GB22521@infradead.org> <48DBFD42.6030307@redhat.com> <20080929141326.GA31781@infradead.org> <48E0E7D4.1090409@sandeen.net> <20080929143749.GA13286@infradead.org> <48E0EA0B.7000701@sandeen.net> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; format=flowed; charset="iso-8859-1"; reply-type=original Content-Transfer-Encoding: 7bit Cc: axboe@kernel.dk, mtk.manpages@googlemail.com, linux-kernel@vger.kernel.org, xfs@oss.sgi.com, dm-devel@redhat.com, viro@ZenIV.linux.org.uk, linux-fsdevel@vger.kernel.org, Andrew Morton , linux-ext4@vger.kernel.org, Ric Wheeler , Oleg Nesterov To: "Eric Sandeen" , "Christoph Hellwig" Return-path: In-Reply-To: <48E0EA0B.7000701@sandeen.net> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com List-Id: linux-ext4.vger.kernel.org Hi, Eric Sandeen wrote: > Christoph Hellwig wrote: >> On Mon, Sep 29, 2008 at 09:36:04AM -0500, Eric Sandeen wrote: >>> Christoph Hellwig wrote: >>>> On Fri, Sep 26, 2008 at 05:52:35PM +0900, Takashi Sato wrote: >>>>> I think that your concern is that the freezer cannot recognize the occurrence >>>>> of a timeout and it continues the backup process and the backup data is >>>>> corrupted finally. >>>> What timeout should happen? the freeze ioctl must not return until the >>>> filesystem is a clean state and all writes are blocked. >>> The suggestion was that *UN*freeze would return ETIMEDOUT if the >>> filesystem had already unfrozen itself, I think. That way you know that >>> the snapshot you just took is worthless, at least. >> >> But why would the filesystem every unfreeze itself? That defeats the >> whole point of freezing it. > > I agree. Was just trying to clarify the above point. > > But there have been what, 12 submissions now, with the unfreeze timeout > in place so it's a persistent theme ;) > > Perhaps a demonstration of just how easy (or not easy) it is to deadlock > a filesystem by freezing the root might be in order, at least. > > And even if it is relatively easy, I still maintain that it is the > administrator's role to not inflict damage on the machine being > administered. There are a lot of potentially dangerous tools at root's > disposal; why this particular one needs a nanny I'm still not quite sure. I think we need the timeout for the case someone dirties so much data with mmap, hence the freeze process is swapped out and cannot unfreeze. Cheers, Takashi