From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C26F9CA9EA0 for ; Fri, 18 Oct 2019 12:55:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 97947222C2 for ; Fri, 18 Oct 2019 12:55:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2393019AbfJRMzZ (ORCPT ); Fri, 18 Oct 2019 08:55:25 -0400 Received: from mga17.intel.com ([192.55.52.151]:45406 "EHLO mga17.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727993AbfJRMzZ (ORCPT ); Fri, 18 Oct 2019 08:55:25 -0400 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga004.fm.intel.com ([10.253.24.48]) by fmsmga107.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 18 Oct 2019 05:55:24 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.67,311,1566889200"; d="scan'208";a="221731964" Received: from jlenoir-mobl.ger.corp.intel.com (HELO localhost) ([10.252.2.252]) by fmsmga004.fm.intel.com with ESMTP; 18 Oct 2019 05:55:22 -0700 Date: Fri, 18 Oct 2019 15:55:21 +0300 From: Jarkko Sakkinen To: Sean Christopherson Cc: linux-sgx@vger.kernel.org Subject: Re: [PATCH for_v23 v3 12/12] x86/sgx: Reinstate per EPC section free page counts Message-ID: <20191018125521.GD4027@linux.intel.com> References: <20191016183745.8226-1-sean.j.christopherson@intel.com> <20191016183745.8226-13-sean.j.christopherson@intel.com> <20191018124942.GC4027@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191018124942.GC4027@linux.intel.com> Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-sgx-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-sgx@vger.kernel.org On Fri, Oct 18, 2019 at 03:49:42PM +0300, Jarkko Sakkinen wrote: > On Wed, Oct 16, 2019 at 11:37:45AM -0700, Sean Christopherson wrote: > > Track the free page count on a per EPC section basis so that the value > > is properly protected by the section's spinlock. > > > > As was pointed out when the change was proposed[*], using a global > > non-atomic counter to track the number of free EPC pages is not safe. > > The order of non-atomic reads and writes are not guaranteed, i.e. > > concurrent RMW operats can write stale data. This causes a variety > > of bad behavior, e.g. livelocks because the free page count wraps and > > causes the swap thread to stop reclaiming. > > > > Signed-off-by: Sean Christopherson > > What is the reason not change it just to atomic? > > For debugging the global is useful because it could be exposed > as a sysfs file. So the regression is that counter updates is read + store (was not btw described in the commit message what the regression you are speaking of, which means that I'm making a guess myself). This means that changing the variable to atomic should be IMHO sufficient. /Jarkko