From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:38180) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bxFTg-0006gM-WA for qemu-devel@nongnu.org; Thu, 20 Oct 2016 11:41:58 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bxFTc-00023k-0N for qemu-devel@nongnu.org; Thu, 20 Oct 2016 11:41:57 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56292) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1bxFTb-00023Y-P0 for qemu-devel@nongnu.org; Thu, 20 Oct 2016 11:41:51 -0400 Date: Thu, 20 Oct 2016 17:41:47 +0200 From: Igor Mammedov Message-ID: <20161020174147.4a548de4@nial.brq.redhat.com> In-Reply-To: <20161020151531.GD5057@thinpad.lan.raisama.net> References: <20161020061301.31372-1-haozhong.zhang@intel.com> <20161020143412.5ea6b564@nial.brq.redhat.com> <20161020132159.GU5057@thinpad.lan.raisama.net> <20161020133353.qfscowwx5bldzvdc@hz-desktop> <20161020134718.GX5057@thinpad.lan.raisama.net> <20161020161735.7d148fd5@nial.brq.redhat.com> <20161020151531.GD5057@thinpad.lan.raisama.net> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] hostmem-file: add a property 'notrunc' to avoid data corruption List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eduardo Habkost Cc: qemu-devel@nongnu.org, Paolo Bonzini , Peter Crosthwaite , Richard Henderson , Xiao Guangrong , kwolf@redhat.com, mreitz@redhat.com On Thu, 20 Oct 2016 13:15:31 -0200 Eduardo Habkost wrote: > On Thu, Oct 20, 2016 at 04:17:35PM +0200, Igor Mammedov wrote: > > On Thu, 20 Oct 2016 11:47:18 -0200 > > Eduardo Habkost wrote: > > > > > On Thu, Oct 20, 2016 at 09:33:53PM +0800, Haozhong Zhang wrote: > > > > On 10/20/16 11:21 -0200, Eduardo Habkost wrote: > > > > > On Thu, Oct 20, 2016 at 02:34:12PM +0200, Igor Mammedov wrote: > > > > > > On Thu, 20 Oct 2016 14:13:01 +0800 > > > > > > Haozhong Zhang wrote: > > > > > > > > > > > > > If a file is used as the backend of memory-backend-file and its size is > > > > > > > not identical to the property 'size', the file will be truncated. For a > > > > > > > file used as the backend of vNVDIMM, its data is expected to be > > > > > > > persistent and the truncation may corrupt the existing data. > > > > > > I wonder if it's possible just skip 'size' property in your case instead > > > > > > 'notrunc' property. That way if size is not present one'd get actual size > > > > > > using get_file_size() and set 'size' to it? > > > > > > And if 'size' is provided and 'size' != file_size then error out. > > > > > > > > > > I think it is valid to start with a zero-size file and then let > > > > > QEMU extend it. > > > > > > > > For vNVDIMM, extending from zero-size file can be valid when a file is > > > > first used. However, it's not valid for the second and following use > > > > of the same file. > > > > > > > > > But I agree we should: 1) make 'size' optional as > > > > > you suggested; 2) never truncate the file to a smaller size. > > > > > > > > > > > > > I will add another patch for this. Is there any way in QEMU to decide > > > > whether a memory-backend-file object is used for vNVDIMM when the > > > > object is being created? Or 'size' can be optional for all kinds of > > > > usages? > > > > > > I believe 'size' can be optional for all usage, because at the > > > moment the memory allocation code asks the backend for a memory > > > region, it is supposed to know desired RAM size from the frontend > > > configuration (-numa, -m, or "size" property of pc-dimm). > > > > nope, currently the size propagates other way around > > from back-end to front-end and not backwards > > I'd say that this is a bug. Frontend size is guest ABI and > shouldn't be overridden by backend configuration if it's > explicitly set. frontend.size is always <= backend.size allocation specified when backend is created (-object/object_add) and front end size if needed/used is <= backend size so far code followed this design.