From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757896AbYHTVr4 (ORCPT ); Wed, 20 Aug 2008 17:47:56 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752903AbYHTVrs (ORCPT ); Wed, 20 Aug 2008 17:47:48 -0400 Received: from jalapeno.cc.columbia.edu ([128.59.29.5]:54921 "EHLO jalapeno.cc.columbia.edu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751545AbYHTVrr (ORCPT ); Wed, 20 Aug 2008 17:47:47 -0400 Message-ID: <48AC8F63.3050500@cs.columbia.edu> Date: Wed, 20 Aug 2008 17:40:51 -0400 From: Oren Laadan Organization: Columbia University User-Agent: Thunderbird 2.0.0.16 (X11/20080724) MIME-Version: 1.0 To: Pavel Machek CC: "Serge E. Hallyn" , containers@lists.linux-foundation.org, Theodore Tso , linux-kernel@vger.kernel.org, Arnd Bergmann , Dave Hansen Subject: Re: [RFC][PATCH 1/4] checkpoint-restart: general infrastructure References: <20080807224033.FFB3A2C1@kernel> <200808081146.54834.arnd@arndb.de> <1218221451.19082.36.camel@nimitz> <200808090013.41999.arnd@arndb.de> <1218234411.19082.58.camel@nimitz> <20080811150703.GA25930@us.ibm.com> <20080814055301.GH6995@ucw.cz> In-Reply-To: <20080814055301.GH6995@ucw.cz> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit X-No-Spam-Score: Local Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Pavel Machek wrote: > Hi! > >>>>> I have to wonder if this is just a symptom of us trying to do this the >>>>> wrong way. We're trying to talk the kernel into writing internal gunk >>>>> into a FD. You're right, it is like a splice where one end of the pipe >>>>> is in the kernel. >>>>> >>>>> Any thoughts on a better way to do this? >>>> Maybe you can invert the logic and let the new syscalls create a file >>>> descriptor, and then have user space read or splice the checkpoint >>>> data from it, and restore it by writing to the file descriptor. >>>> It's probably easy to do using anon_inode_getfd() and would solve this >>>> problem, but at the same time make checkpointing the current thread >>>> hard if not impossible. >>> Yeah, it does seem kinda backwards. But, instead of even having to >>> worry about the anon_inode stuff, why don't we just put it in a fs like >>> everything else? checkpointfs! >> One reason is that I suspect that stops us from being able to send that >> data straight to a pipe to compress and/or send on the network, without >> hitting local disk. Though if the checkpointfs was ram-based maybe not? >> >> As Oren has pointed out before, passing in an fd means we can pass a >> socket into the syscall. > > If you do pass a socket, will it handle blocking correctly? Getting > deadlocked task would be bad. What happens if I try to snapshot into > /proc/self/fd/0 ? Or maybe restore from /proc/cmdline? Hmmm... these are good points. Keep in mind that our principal goal is to checkpoint a whole container, rather then a task to checkpoint itself (which is a by-product). Of course your comments apply to a whole container as well. In both cases, I don't think that blocking on a socket is a problem; the checkpointer will enter a TASK_INTERRUPTIBLE state. Where is the deadlock ? Writing or reading to/from /proc/self/... likewise - the programmer must understand the implications, or the program won't work as expected. I don't see a possible deadlock here, though. For example - writing to /proc/self/fd/0 is ok; the state of fd[0] of that task will be captured at some point in the middle of the checkpoint, so after restart one cannot assume anything about the file position; the rest should work. Oren.