From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with ESMTP id 7133E6B0044 for ; Thu, 18 Dec 2008 10:03:36 -0500 (EST) Received: from d01relay02.pok.ibm.com (d01relay02.pok.ibm.com [9.56.227.234]) by e3.ny.us.ibm.com (8.13.1/8.13.1) with ESMTP id mBIF4gpK007308 for ; Thu, 18 Dec 2008 10:04:42 -0500 Received: from d01av01.pok.ibm.com (d01av01.pok.ibm.com [9.56.224.215]) by d01relay02.pok.ibm.com (8.13.8/8.13.8/NCO v9.1) with ESMTP id mBIF5X7a189196 for ; Thu, 18 Dec 2008 10:05:33 -0500 Received: from d01av01.pok.ibm.com (loopback [127.0.0.1]) by d01av01.pok.ibm.com (8.12.11.20060308/8.13.3) with ESMTP id mBIF5Wgx004008 for ; Thu, 18 Dec 2008 10:05:33 -0500 Subject: Re: [RFC v11][PATCH 05/13] Dump memory address space From: Dave Hansen In-Reply-To: <494A2F94.2090800@cs.columbia.edu> References: <1228498282-11804-1-git-send-email-orenl@cs.columbia.edu> <1228498282-11804-6-git-send-email-orenl@cs.columbia.edu> <4949B4ED.9060805@google.com> <494A2F94.2090800@cs.columbia.edu> Content-Type: text/plain Date: Thu, 18 Dec 2008 07:05:20 -0800 Message-Id: <1229612720.17206.505.camel@nimitz> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Oren Laadan Cc: Mike Waychison , jeremy@goop.org, arnd@arndb.de, linux-api@vger.kernel.org, containers@lists.linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Linux Torvalds , Alexander Viro , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar List-ID: On Thu, 2008-12-18 at 06:10 -0500, Oren Laadan wrote: > >> + mutex_lock(&mm->context.lock); > >> + > >> + hh->ldt_entry_size = LDT_ENTRY_SIZE; > >> + hh->nldt = mm->context.size; > >> + > >> + cr_debug("nldt %d\n", hh->nldt); > >> + > >> + ret = cr_write_obj(ctx, &h, hh); > >> + cr_hbuf_put(ctx, sizeof(*hh)); > >> + if (ret < 0) > >> + goto out; > >> + > >> + ret = cr_kwrite(ctx, mm->context.ldt, > >> + mm->context.size * LDT_ENTRY_SIZE); > > > > Do we really want to emit anything under lock? I realize that this > > patch goes and does a ton of writes with mmap_sem held for read -- is > > this ok? > > Because all tasks in the container must be frozen during the checkpoint, > there is no performance penalty for keeping the locks. Although the object > should not change in the interim anyways, the locks protects us from, e.g. > the task unfreezing somehow, or being killed by the OOM killer, or any > other change incurred from the "outside world" (even future code). > > Put in other words - in the long run it is safer to assume that the > underlying object may otherwise change. > > (If we want to drop the lock here before cr_kwrite(), we need to copy the > data to a temporary buffer first. If we also want to drop mmap_sem(), we > need to be more careful with following the vma's.) > > Do you see a reason to not keeping the locks ? Mike, although we're doing writes of the checkpoint file here, the *mm* access is read-only. We only need really mmap_sem for write if we're creating new VMAs, which we only do on restore. Was there an action taken on the mm that would require a write that we missed? Oren, I never considered the locking overhead, either. The fact that the processes are frozen is very, very important here. The code is fine as it stands because this *is* a very simple way to do it. But, this probably deserves a comment. -- Dave -- 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