From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751977AbdHCNQG (ORCPT ); Thu, 3 Aug 2017 09:16:06 -0400 Received: from mga03.intel.com ([134.134.136.65]:46616 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751953AbdHCNQE (ORCPT ); Thu, 3 Aug 2017 09:16:04 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.41,316,1498546800"; d="scan'208";a="885989587" Message-ID: <598322B6.8090204@intel.com> Date: Thu, 03 Aug 2017 21:18:46 +0800 From: Wei Wang User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Michal Hocko CC: linux-kernel@vger.kernel.org, linux-mm@kvack.org, virtualization@lists.linux-foundation.org, mst@redhat.com, zhenwei.pi@youruncloud.com, akpm@linux-foundation.org, dave.hansen@intel.com, mawilcox@microsoft.com Subject: Re: [PATCH RESEND] mm: don't zero ballooned pages References: <1501761557-9758-1-git-send-email-wei.w.wang@intel.com> <20170803125409.GT12521@dhcp22.suse.cz> In-Reply-To: <20170803125409.GT12521@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/03/2017 08:54 PM, Michal Hocko wrote: > On Thu 03-08-17 19:59:17, Wei Wang wrote: >> This patch is a revert of 'commit bb01b64cfab7 ("mm/balloon_compaction.c: >> enqueue zero page to balloon device")' >> >> Ballooned pages will be marked as MADV_DONTNEED by the hypervisor and >> shouldn't be given to the host ksmd to scan. > I find MADV_DONTNEED reference still quite confusing. What do you think > about the following wording instead: > " > Zeroying ballon pages is rather time consuming, especially when a lot of > pages are in flight. E.g. 7GB worth of ballooned memory takes 2.8s with > __GFP_ZERO while it takes ~491ms without it. The original commit argued > that zeroying will help ksmd to merge these pages on the host but this > argument is assuming that the host actually marks balloon pages for ksm > which is not universally true. So we pay performance penalty for > something that even might not be used in the end which is wrong. The > host can zero out pages on its own when there is a need. > " I think it looks good. Thanks. >> Therefore, it is not >> necessary to zero ballooned pages, which is very time consuming when >> the page amount is large. The ongoing fast balloon tests show that the >> time to balloon 7G pages is increased from ~491ms to 2.8 seconds with >> __GFP_ZERO added. So, this patch removes the flag. > The only reason why unconditional zeroying makes some sense is the > data leak protection (guest doesn't want to leak potentially sensitive > data to a malicious guest). I am not sure such a thread applies here > though. I think the unwashed contents left in the balloon pages (also free pages) should be treated non-confidential - if the guest application has confidential content in its memory, the application itself should zero that before giving back that memory to the guest kernel. Best, Wei